Image forming apparatus

ABSTRACT

An image forming apparatus includes a primary transfer bias adjuster to adjust, in a second mode, a primary transfer bias by a correction amount in accordance with a degree of deterioration of a developing agent detected by a first developing agent condition detector so as to reduce a primary transfer current of the primary transfer bias upon transferring a toner image from a first image bearing member onto an intermediate transfer member, and to adjust, in a first mode, the primary transfer bias by a correction amount less than the correction amount in the second mode or not to adjust the primary transfer bias in accordance with the degree of deterioration of the developing agent detected by the first developing agent condition detector upon primarily transferring the toner image from the first image bearing member onto the intermediate transfer member.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is based on and claims priority pursuant to 35U.S.C. §119 from Japanese Patent Application Nos. 2013-052496, filed onMar. 14, 2013, and 2013-052813, filed on Mar. 15, 2013, both in theJapan Patent Office, which are hereby incorporated herein by referencein their entirety.

BACKGROUND

1. Technical Field

Exemplary aspects of the present invention generally relate to an imageforming apparatus, such as a copier, a facsimile machine, a printer, ora multi-functional system including a combination thereof, and moreparticularly to a tandem-type image forming apparatus.

2. Description of the Related Art

Known image forming apparatuses using an intermediate transfer methodare generally equipped with a plurality of image bearing membersarranged in tandem along a moving direction of an intermediate transfermember. Toner images are formed on the plurality of image bearingmembers and then transferred primarily onto the intermediate transfermember. Subsequently, the toner images are secondarily transferred ontoa recording medium. In such image forming apparatuses using theintermediate transfer method, toner images are formed on at least twoimage bearing members and the toner images are primarily transferredonto the intermediate transfer member such that they are superimposedone atop the other, forming a composite toner image, which is thentransferred onto a recording medium in a multi-color mode (first controlmode). Alternatively, a toner image is formed on a single image bearingmember, and primarily transferred onto the intermediate transfer member,and then transferred onto a recording medium in a single-color mode(second control mode).

In this type of image forming apparatuses, a developing agentdeteriorates with time, causing insufficient charging of overall toner,which results in an image defects, such as roughness in a halftoneimage. To address this difficulty, in JP-2009-168925-A, a degree ofdegradation of an imaging device that forms the toner image on the imagebearing member disposed at an extreme downstream end in the movingdirection of the intermediate transfer member is detected. When thedegree of deterioration reaches a certain level, a level of secondarytransfer bias is reduced.

According to JP-2009-168925-A, in a case in which the charge amount oftoner is generally low, flow of electrical charge due to movement of thetoner is small at a secondary transfer portion, hence generatingelectric discharge at the secondary transfer portion and resulting in arough image. In such a case, the secondary transfer bias is reduced sothat generation of electric discharge at the secondary transfer portionis suppressed and the roughness in a low-density image is thus reduced.

In this type of image forming apparatus, that is, i.e., a tandem-typeimage forming apparatus, the plurality of image bearing members isdisposed along the moving direction of a transfer medium such as arecording medium and the intermediate transfer member, and the tonerimages formed on the image bearing members are transferred onto thetransfer medium. The toner images on the image bearing members aretransferred onto the transfer medium by applying a transfer bias from atransfer device. A ratio (transfer rate) of toner constituting the tonerimage to be transferred onto the transfer medium depends on the chargeamount of toner and the level of the transfer bias.

JP-H05-158357-A proposes a monochrome image forming apparatus thatdirectly transfers a toner image from a single image bearing member,i.e., a photosensitive drum onto a recording medium (transfer medium).This image forming apparatus measures a number of printed sheets or anumber of printed sheets corresponding to mixing of the developingagent. Based on the measurement, the transfer bias is adjusted to havean optimum transfer current which is inversely proportional to changesin the charge amount of toner with time. In this configuration, evenwhen the charge amount of toner is generally low due to deterioration ofthe developing agent with time, adjustment of the transfer currentaccording to the deterioration of the developing agent suppresses imagedefects attributed to the deterioration of the developing agent.

In the image forming apparatus using the intermediate transfer method,when the developing agent deteriorates with time, causing the chargeamount of overall toner to drop low, primary transfer at the primarytransfer portion may be affected. That is, even before secondarytransfer, the toner image may be degraded. It is difficult to preventsuch degradation of image quality only by adjusting the secondarytransfer bias as described above. In order to suppress degradation ofimage quality attributed to the deterioration of the developing agentwith time, causing the charge amount of overall toner to drop low, itmay be necessary to adjust the primary transfer bias to be applied tothe primary transfer portion.

FIG. 12 is a graph showing relations between a primary transfer rate anda primary transfer current in an initial state in which the developingagent has not deteriorated yet.

FIG. 13 is a graph showing relations between the primary transfer rateand the primary transfer current when the developing agent deterioratedwith time.

FIGS. 12 and 13 show results of measurement on a 5% band image and a 95%band image. As illustrated in FIG. 14, the 5% band image is aband-shaped toner pattern having a width which corresponds to 5% of anentire imaging width, formed substantially in the center in a mainscanning direction and extending in a sub-scanning direction (sheetmoving direction). As illustrated in FIG. 15, the 95% band image is aband-shaped toner pattern having a width which corresponds to 95% of theentire imaging width, formed at one side in the main scanning direction,and extending in the sub-scanning direction (sheet moving direction).

In the initial state in which the developing agent has not yetdeteriorated, the relations between the primary transfer rate and theprimary transfer current for the 5% band image and the 95% band imagelook like the one shown in FIG. 12, in which there is a peak in theprimary transfer rate for both images. An optimum level of the primarytransfer current for achieving a highest possible primary transfer ratewithin a range in which substantially the same primary transfer rate isachieved for both the 5% band image and the 95% band image is similar toor the same value such as shown in FIG. 12. Generally, in most cases,the primary transfer current is set to the value shown in FIG. 12.

By contrast, when the developing agent deteriorated with time, therelations between the primary transfer rate and the primary transfercurrent for the 5% band image and the 95% band image look like the oneshown in FIG. 13. In FIG. 13, the highest peak of the primary transferrate for the 95% band image shifts largely toward a low primary transfercurrent (absolute value) side as compared with the initial state. Inthis case, the optimum primary transfer current for achieving thehighest possible primary transfer rate within the range in whichsubstantially the same primary transfer rate is achieved for both the 5%band image and the 95% band image is similar to or the same value suchas shown in FIG. 13. As shown in FIG. 13, the absolute value of theoptimum primary transfer current decreases with time as compared withthe initial state.

The reason for the decrease in the optimum primary transfer current(absolute value) with the deteriorated developing agent with time isassumed as follows.

During which an amount of toner moving from the image bearing member tothe intermediate transfer belt at the primary transfer portion increaseswith an increase in the primary transfer bias, flow of electric currentcaused by the movement of toner increases, hence increasing the primarytransfer current. After the amount of movement of toner reaches a stateof saturation, the flow of electric current caused by the movement oftoner stops increasing. In this case, electrical discharge at theprimary transfer portion increases in accordance with an increase in theprimary transfer bias. Thus, once the amount of movement of tonerreaches the state of saturation, the primary transfer current keepsincreasing in accordance with generation of the electrical discharge.

On the other hand, with an increase in the electrical discharge, theprimary transfer rate decreases. That is, after the amount of movementof toner reaches the state of saturation, the primary transfer ratedecreases as the primary transfer current increases. As a result, asshown in FIGS. 12 and 13, there is a highest peak in the primarytransfer rate in the relations between the primary transfer current andthe primary transfer rate. When the developing agent deteriorates withtime, the charge amount of toner is relatively low overall. In such acase, the flow of electric current caused by the movement of the tonerat the primary transfer portion is less than that in the initial stateso that the primary transfer current when the amount of move of tonerreaches the state of saturation is less than that in the initial state.As a result, when the developing agent deteriorated with time, theoptimum primary transfer current (absolute value) to achieve the optimumprimary transfer rate is lower than that in the initial state.

However, the present inventors recognized that reducing uniformly theprimary transfer current (absolute value) flowing through the primarytransfer portion in accordance with the degree of deterioration of thedeveloping agent may rather degrade the image quality. Morespecifically, as will be described later, although reducing the primarytransfer current enhances the primary transfer rate, reducing theprimary transfer current reduces the secondary transfer rate. Therefore,reducing the primary transfer current does not necessarily improve theimage quality. Rather, it may reduce the image quality. It is also knownthat a rate of decrease in the secondary transfer rate after correctionof the primary transfer current in the single-color mode (second controlmode) is greater in the multi-color mode (first control mode). Thus, ifthe amount of correction of the primary transfer current in themulti-color mode (first control mode) is the same as or similar to thatin the single-color mode (second control mode), the image quality isdegraded more easily.

In the first control mode, the toner images formed on the plurality ofimage bearing members are transferred onto the intermediate transfermember such that they are superimposed one atop the other, forming acomposite toner image. The composite toner image thus obtained istransferred secondarily from the intermediate transfer member to arecording medium. By contrast, in the second control mode in which onlyone of the image bearing members (hereinafter referred to as downstreamimage bearing member) used in the multi-color mode is used, one tonerimage without other toner images superimposed thereon is transferredsecondarily from the intermediate member to the recording medium. As aresult, an amount of toner to be transferred secondarily to therecording medium at the secondary transfer portion, in general, isgreater in the first control mode than in the second control mode.Therefore, an optimum secondary transfer bias to achieve an optimumsecondary transfer rate is greater in the first control mode than in thesecond control mode. Thus, the secondary transfer bias is greater in thefirst control mode than in the second control mode.

At this time, if the primary transfer bias is adjusted so that theprimary transfer current is reduced in accordance with the degree ofdeterioration of the developing agent, the primary transfer rate isenhanced. However, since the charge amount of overall toner isrelatively low due to deterioration of the developing agent and hencethe primary transfer current is relatively small, the charge amount oftoner at the secondary transfer portion is even lower than beforecorrection. Degradation of image quality attributed to the decrease inthe charge amount of toner at the secondary transfer portion is greaterin the first control mode in which the secondary transfer bias isrelatively high than in the second control mode in which the secondarytransfer bias is relatively low.

FIG. 16 is a graph showing relations between the secondary transfer rateand the secondary transfer current associated with a toner image on thedownstream image bearing member.

The relations between the secondary transfer rate and the secondarytransfer current may be considered as having substantially the samerelations as between the primary transfer current and the primarytransfer rate. That is, during which the amount of toner moving from theimage bearing member to the intermediate transfer belt at the secondarytransfer portion increases with an increase in the secondary transferbias, flow of electric current caused by the movement of tonerincreases, hence increasing the secondary transfer current. By contrast,after the amount of move of toner reaches a state of saturation, theflow of electric current caused by the movement of toner stopsincreasing. Consequently, the electrical discharge at the secondarytransfer portion increases in accordance with an increase in thesecondary transfer bias. In this case, the primary transfer currentincreases with an increase in the electrical discharge while thesecondary transfer rate decreases with the increase in the electricaldischarge. FIG. 16 shows the resulting relations between the secondarytransfer current and the secondary transfer rate.

The secondary transfer current in the first control mode is set toachieve a highest possible secondary transfer rate within a range inwhich the secondary transfer rates for each of the plurality of tonerimages constituting the composite toner image are approximately the same(that is, none of the secondary transfer rates has a low value relativeto all the other ratios). The toner image to be transferred primarilyfrom the image bearing member disposed in the upstream side in themoving direction of the intermediate transfer member among the pluralityof toner images constituting the composite toner image is charged upwith the primary transfer current when passing through the primarytransfer portion in the downstream therefrom. As a result, the chargeamount of toner in the secondary transfer portion is higher than that ofthe toner image to be transferred primarily from the downstream imagebearing member.

In a case in which the secondary transfer current for transferringsecondarily the plurality of toner images having different chargeamounts all at once is determined as described above, for the tonerimage with a relatively low charge amount (the toner image transferredprimarily from the downstream image bearing member), the secondarytransfer current is set to a value higher than a value (peak value)achieving the maximum secondary transfer rate. By contrast, in thesecond control mode using one image bearing member, i.e., the downstreamimage bearing member, because there is one toner image, the secondarytransfer current is set to achieve the optimum secondary transfer ratefor the toner image.

In a case in which the primary transfer bias is adjusted to reduce theprimary transfer current for the downstream image bearing member inaccordance with the rate of deterioration of the developing agent in thefirst control mode and in the second control mode in which therespective secondary transfer current is determined in a mannerdescribed above, the primary transfer current after correction is lowfor the toner image on the downstream image bearing member, resulting ina lower charge amount of toner in the secondary transfer portion thanbefore correction. At this time, as the charge amount of toner in thesecondary transfer portion decreases, the flow of electric currentcaused by the movement of the toner at the secondary transfer portion isreduced. Thus, the secondary transfer current when the amount of move oftoner reaches the state of saturation (i.e., when the secondary transferrate reaches its peak) is less than that before correction. As a result,the relations between the secondary transfer current and the secondarytransfer rate after correction of the primary transfer bias indicated bya broken line in FIG. 16 shift towards the lower secondary transfercurrent side as compared with the relations before correction indicatedby a solid line in FIG. 16.

As shown in FIG. 16, a rate of change in the secondary transfer raterelative to the change in the secondary transfer current tends toincrease as the secondary transfer current shifts away from the peakvalue capable of achieving the maximum secondary transfer rate. The setvalue for the secondary transfer current in the first control modebefore correction is at a higher secondary transfer current side thanthe peak value capable of achieving the maximum secondary transfer rateas described above. Consequently, when the peak value shifts toward alower secondary transfer current side due to correction of the primarytransfer bias, the set value for the secondary transfer current aftercorrection shifts even further away from the peak value capable ofachieving the maximum secondary transfer rate. As a result, thecorrection of the primary transfer bias causes the secondary transferrate to drop significantly.

In the tandem-type image forming apparatus in which the toner imagesformed on the plurality of image bearing members are transferred onto atransfer medium such that they are superimposed one atop the other,preferably, the transfer current is corrected in accordance withparameters in correlation with the degree of deterioration of thedeveloping agent such as the number of printed sheets as proposed inJP-05-158357-A.

The present inventors have recognized, however, that the effect ofcorrection of the transfer current in accordance with the degree ofdeterioration of the developing agent relative to degradation of imagequality differs depending on the toner images. Furthermore, in thesingle-drum type image forming apparatus in which the plurality of tonerimages is formed on the single image bearing member and transferredsequentially onto a transfer medium, when the transfer current iscorrected in accordance with the degree of deterioration of thedeveloping agent as described above, the effect of correction of thetransfer current on degradation of image quality differs depending onthe toner images.

The present inventors have also recognized that in the tandem-type imageforming apparatus, the effect of correction of the transfer current inaccordance with the degree of deterioration of the developing agentrelative to the degradation of image quality differs depending on theimage bearing members because the volume resistivity of toners in thedeveloping agents used to form the toner images on the image bearingmembers differs. That is, depending on the volume resistivity of toners,the rate of change in the optimum value of the transfer current inaccordance with the deterioration of the developing agent is different.

An apparent electrostatic capacity of toner having a relatively lowvolume resistivity decreases as the electrical resistivity decreases sothat the toner is difficult to keep relatively the charge. When thecharging ability of the toner decreases due to deterioration of thedeveloping agent, the decrease in the charge amount of toner having thelow volume resistivity is relatively large. As a result, even when theoptimum transfer current is set corresponding to the volume resistivityof the respective toner in the initial state, with deterioration of thedeveloping agent after extended use, the rate of change in the optimumtransfer current from the initial state is greater in the toner with thelow volume resistivity.

Therefore, if the same correction is performed on the transfer currentin accordance with the degree of deterioration of the developing agenton the basis of the toner having a high volume resistivity, adequatecorrection is not performed for the toner with a low volume resistivityand hence degradation of image quality is not suppressed sufficiently.By contrast, if the same correction is performed on the transfer currentin accordance with the degree of deterioration of the developing agenton the basis of the toner having a low volume resistivity,overcorrection occurs for the toner with a high volume resistivity andhence the transfer rate is not improved sufficiently, resulting in thedegradation of image quality.

SUMMARY

In view of the foregoing, in an aspect of this disclosure, there isprovided a novel image forming apparatus including an intermediatetransfer member to move in a first direction; a plurality of imagebearing members to bear toner images thereon, the plurality of imagebearing members disposed along the first direction; a plurality of tonerimage forming devices to form the toner images on the plurality of imagebearing members using different developing agents; a primary transferdevice to apply a primary transfer bias to primarily transfer each ofthe toner images formed on the plurality of image bearing members onto asurface of the intermediate transfer member to form a composite tonerimage; a secondary transfer device to apply a secondary transfer bias tosecondarily transfer the composite toner image formed on theintermediate transfer member to a recording medium; a controller toselectively control image forming operation between a first mode and asecond mode such that in the first mode the composite toner image isformed using at least two of the plurality of image bearing membersincluding a first image bearing member and a second image bearing memberand after the composite toner image is formed on the intermediatetransfer member the secondary transfer bias is applied to secondarilytransfer the composite toner image from the intermediate transfer memberto the recording medium, and in the second mode the toner image isformed on the first image bearing member used in the first mode which isdisposed downstream from the second image bearing member in the firstdirection and after the toner image is primarily transferred from thefirst image bearing member onto the intermediate transfer member thesecondary transfer bias less than that in the first mode is applied totransfer the toner image from the intermediate transfer member to therecording medium; a first developing agent condition detector to detecta degree of deterioration of a developing agent used to form the tonerimage on the first image bearing member; and a primary transfer biasadjuster to adjust, in the second mode, the primary transfer bias by acorrection amount in accordance with the degree of deterioration of thedeveloping agent detected by the first developing agent conditiondetector so as to reduce a primary transfer current of the primarytransfer bias upon transferring the toner image from the first imagebearing member onto the intermediate transfer member, and to adjust, inthe first mode, the primary transfer bias by a correction amount lessthan the correction amount in the second mode or not to adjust theprimary transfer bias in accordance with the degree of deterioration ofthe developing agent detected by the first developing agent conditiondetector upon primarily transferring the toner image from the firstimage bearing member onto the intermediate transfer member.

According to another aspect, an image forming apparatus includes animage bearing member to rotate; an intermediate transfer member to movein a first direction; a plurality of toner image forming devices to formsequentially and overlappingly a plurality of toner images usingdifferent developing agents on a surface of the image bearing member toform a composite toner image; a primary transfer device to apply aprimary transfer bias to primarily transfer the composite toner imageformed on the image bearing member onto a surface of the intermediatetransfer member; a secondary transfer device to apply a secondarytransfer bias to secondarily transfer the composite toner image havingbeen primarily transferred on the intermediate transfer member onto arecording medium; a controller to selectively control image formingoperation between a first mode and a second mode such that in the firstmode the composite toner image is formed using at least two of theplurality of toner image forming devices including a first toner imageforming device and a second toner image forming device and after thecomposite toner image is primarily transferred onto the intermediatetransfer member the secondary transfer bias is applied to secondarilytransfer the composite toner image from the intermediate transfer memberonto the recording medium, and in the second mode the first toner imageforming device used in the first mode forms the toner image which istransferred after the toner image formed by the toner image formingdevice other than the first toner image forming device is transferredand after the toner image formed by the first toner image forming deviceis primarily transferred onto the intermediate transfer member thesecondary transfer bias less than that in the first mode is applied totransfer the toner image from the intermediate transfer member onto therecording medium; a developing agent condition detector to detect adegree of deterioration of a developing agent used to form the tonerimage on the first toner image forming device; and a primary transferbias adjuster to adjust the primary transfer bias by a correction amountin accordance with the degree of deterioration of the developing agentdetected by the developing agent condition detector so as to reduce aprimary transfer current of the primary transfer bias upon transferringthe toner image formed by the first toner image forming device in thesecond mode, and to adjust the primary transfer bias by the correctionamount less than the correction amount in the second mode or not toadjust the primary transfer bias in accordance with the degree ofdeterioration of the developing agent detected by the developing agentcondition detector upon primarily transferring the toner image formed bythe first toner image forming member in the first mode.

According to still another aspect, an image forming apparatus includes aplurality of image bearing members to rotate in a first direction; aplurality of toner image forming devices to form a toner image on asurface of each of the plurality of image bearing members withdeveloping agents including toners having different volumeresistivities; a plurality of transfer devices to apply a transfer biasto transfer the toner images formed on the plurality of image bearingmembers onto a transfer medium to form a composite toner image; adeveloping agent condition detector to detect a degree of deteriorationof the developing agents; and a transfer current adjuster to adjust atransfer current of the transfer bias by a correction amount inaccordance with the degree of deterioration of the developing agentdetected by the developing agent condition detector upon transferringthe toner images formed on at least two image bearing members, the tonerimages being formed with the developing agents including the tonershaving different volume resistivities. The correction amount isdifferent between the at least two image bearing members.

According to still another aspect, the image forming apparatus includesan image bearing member to rotate in a first direction; a plurality oftoner image forming devices to form toner images on a surface of theimage bearing member using different developing agents including tonershaving different volume resistivities; a transfer device to apply atransfer bias to transfer sequentially the toner images formed on theplurality of image bearing members onto a transfer medium to form acomposite toner image; a developing agent condition detector to detect adegree of deterioration of the developing agents; and a transfer currentadjuster to adjust a transfer current of the transfer bias by acorrection amount in accordance with the degree of deterioration of thedeveloping agents detected by the developing agent condition detectorupon transferring at least two toner images formed with the developingagents including the toners having different volume resistivities. Thecorrection amount is different between the at least two toner images.

The aforementioned and other aspects, features and advantages would bemore fully apparent from the following detailed description ofillustrative embodiments, the accompanying drawings and the associatedclaims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be more readily obtained as the same becomesbetter understood by reference to the following detailed description ofillustrative embodiments when considered in connection with theaccompanying drawings, wherein:

FIG. 1 is a schematic diagram illustrating a printer as an example of animage forming apparatus according to an illustrative embodiment of thepresent disclosure;

FIG. 2 is a schematic diagram illustrating an image forming unit for thecolor yellow employed in the image forming apparatus of FIG. 1;

FIG. 3 is a graph showing relations of a secondary transfer current anda secondary transfer rate for a black toner image, a cyan toner image,and a two-color toner image with cyan and magenta when a primarytransfer current is changed for the black toner image;

FIG. 4 is a graph showing relations of the primary transfer current forthe black toner image and a charge amount of toner for each toner imagebefore secondary transfer;

FIG. 5 is a flowchart showing steps in a process of determination of anamount of correction in accordance with a degree of deterioration of adeveloping agent;

FIG. 6 is a table showing an example of set values of the primarytransfer current for each color in accordance with the degree ofdeterioration of the developing agent and set values of the secondarytransfer current;

FIG. 7 is a table showing another example of set values of the primarytransfer current for each color in accordance with the degree ofdeterioration of the developing agent and set values of the secondarytransfer current;

FIG. 8 is a graph showing an example of a change in the secondarytransfer current setting according to a variation;

FIG. 9 is a table showing results of evaluation of image density whenthe degree of deterioration of the developing agent is in an initialstate;

FIG. 10 is a table showing results of evaluation of image density whenthe degree of deterioration (time group) of the developing agent is in“ELAPSED TIME 1”;

FIG. 11 is a table showing results of evaluation of image density whenthe degree of deterioration (time group) of the developing agent is in“ELAPSED TIME 2”;

FIG. 12 is a graph showing relations between the primary transfer rateand the primary transfer current in the initial state in which thedeveloping agent has not deteriorated yet;

FIG. 13 is a graph showing relations between the primary transfer rateand the primary transfer current when the developing agent hasdeteriorated with time;

FIG. 14 is a conceptual diagram illustrating a 5% band image;

FIG. 15 is a conceptual diagram illustrating a 95% band image;

FIG. 16 is a graph showing relations between the secondary transfer rateand the secondary transfer current associated with a toner image on adownstream image bearing member;

FIG. 17 is a schematic diagram illustrating an example of a single-drumtype image forming apparatus according to an illustrative embodiment ofthe present disclosure;

FIG. 18 is a graph showing relations between a number of sheets (numberof printed sheets) on which an image is formed and the charge amount oftoner (Q/M);

FIG. 19 is a graph showing relations between a traveling distance of adeveloping agent and the charge amount of toner (Q/M);

FIG. 20 is a graph showing relations between the degree of deteriorationof the developing agent and the charge amount of toner (Q/M) accordingto an illustrative embodiment of the present disclosure;

FIG. 21 is a flowchart showing steps in a process of determination of anenvironmental correction amount (environment correction coefficient)according to an illustrative embodiment of the present disclosure;

FIG. 22 is a flowchart showing steps in a process of determination of anelapsed time correction amount (elapsed time correction coefficient)according to an illustrative embodiment of the present disclosure;

FIG. 23 is a flowchart showing another example of steps in a process ofdetermination of the elapsed time correction amount (elapsed timecorrection coefficient) according to an illustrative embodiment of thepresent disclosure;

FIG. 24 is a flowchart showing another example of steps in a process ofdetermination of the elapsed time correction amount (elapsed timecorrection coefficient) according to an illustrative embodiment of thepresent disclosure;

FIG. 25 is a flowchart showing steps in a process of determination ofthe elapsed time correction amount (elapsed time correction coefficient)according to a variation 1 of the second illustrative embodiment;

FIG. 26 is a table showing an example of environmental coefficients foreach environment group corresponding to the degree of deterioration ofthe developing agent according to a variation 5;

FIG. 27 is a table showing an example of relations between an appliedvoltage (detection voltage) of the primary transfer roller and anelectrical resistivity of the primary transfer roller;

FIG. 28 is a table showing an example of relations between theelectrical resistivity of the primary transfer roller and a primarytransfer current (an optimum current) capable of achieving a maximumprimary transfer rate;

FIG. 29 is a table showing an example of relations between a detectedprimary transfer voltage and the elapsed time correction coefficientafter being changed in accordance with the detected primary transfervoltage; and

FIG. 30 is a schematic diagram illustrating an example of an imageforming apparatus using a direct-transfer method according to anillustrative embodiment of the present disclosure.

DETAILED DESCRIPTION

A description is now given of illustrative embodiments of the presentinvention. It should be noted that although such terms as first, second,etc. may be used herein to describe various elements, components,regions, layers and/or sections, it should be understood that suchelements, components, regions, layers and/or sections are not limitedthereby because such terms are relative, that is, used only todistinguish one element, component, region, layer or section fromanother region, layer or section. Thus, for example, a first element,component, region, layer or section discussed below could be termed asecond element, component, region, layer or section without departingfrom the teachings of this disclosure.

In addition, it should be noted that the terminology used herein is forthe purpose of describing particular embodiments only and is notintended to be limiting of this disclosure. Thus, for example, as usedherein, the singular forms “a”, “an” and “the” are intended to includethe plural forms as well, unless the context clearly indicatesotherwise. Moreover, the terms “includes” and/or “including”, when usedin this specification, specify the presence of stated features,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof.

In describing illustrative embodiments illustrated in the drawings,specific terminology is employed for the sake of clarity. However, thedisclosure of this patent specification is not intended to be limited tothe specific terminology so selected, and it is to be understood thateach specific element includes all technical equivalents that have thesame function, operate in a similar manner, and achieve a similarresult.

In a later-described comparative example, illustrative embodiment, andalternative example, for the sake of simplicity, the same referencenumerals will be given to constituent elements such as parts andmaterials having the same functions, and redundant descriptions thereofomitted.

Typically, but not necessarily, paper is the medium from which is made asheet on which an image is to be formed. It should be noted, however,that other printable media are available in sheet form, and accordinglytheir use here is included. Thus, solely for simplicity, although thisDetailed Description section refers to paper, sheets thereof, paperfeeder, etc., it should be understood that the sheets, etc., are notlimited only to paper, but include other printable media as well.

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views, exemplaryembodiments of the present patent application are described.

FIG. 1 is a schematic diagram illustrating a printer as an example of animage forming apparatus according to an illustrative embodiment of thepresent disclosure.

FIG. 2 is a schematic diagram illustrating an image forming unit for thecolor yellow employed in the image forming apparatus of FIG. 1.

As illustrated in FIG. 1, the image forming apparatus includes fourimage forming units 1Y, 1C, 1M, and 1K that form toner images of yellow,cyan, magenta, and black, respectively. It is to be noted that thesuffixes Y, C, M, and K denote colors yellow, cyan, magenta, and black,respectively. To simplify the description, these suffixes are omittedherein, unless otherwise specified. The image forming units 1Y, 1C, 1M,and 1K all have the same configuration as all the others, differing onlyin the color of image formation substance, i.e., developing agentemployed. Thus, a description is provided of the image forming unit 1Yfor forming a toner image of yellow as a representative example of theimage forming units.

As illustrated in FIG. 2, the image forming unit 1Y for forming a tonerimage of the color yellow includes a photosensitive drum unit 2Yincluding a drum-shaped photosensitive drum 3Y (hereinafter referred toas photosensitive drum) serving as a latent image bearing member and adevelopment device 7Y that develops a latent image formed on thephotosensitive drum 3Y. The photosensitive drum unit 2Y and thedevelopment device 7Y constitute a single integrated unit as the imageforming unit 1Y detachably attachable relative to a main body of theimage forming apparatus. When the image forming unit 1Y is detached fromthe main body, the development device 7Y is detachable from thephotosensitive drum unit 2Y.

An optical writing unit 20 as a latent image writing device for writinga latent image on the photosensitive drums 3Y, 3C, 3M, and 3K isdisposed substantially below the image forming units 1Y, 1C, 1M, and 1K.After the photosensitive drums 3Y, 3C, 3M, and 3K are uniformly charged,the optical writing unit 20 illuminates the photosensitive drums 3Y, 3C,3M, and 3K of the image forming units 1Y, 1C, 1M, and 1K with laserlight L based on image information. Accordingly, electrostatic latentimages for the colors yellow, cyan, magenta, and black are formed on thephotosensitive drums 3Y, 3C, 3M, and 3K, respectively. The opticalwriting unit 20 includes a polygon mirror 21, a plurality of opticallenses, and mirrors. The laser light L projected from a light source isdeflected by the polygon mirror driven to rotate by a polygon motor. Thedeflected light, then, strikes the optical lenses and mirrors, therebyscanning the photosensitive drums 3Y, 3C, 3M, and 3K. Alternatively,optical scanning may be performed by using an LED array.

As illustrated in FIG. 1, a first sheet cassette 31 and a second sheetcassette 32 storing a stack of recording media P are vertically disposedbelow the optical writing unit 20. The first sheet cassette 31 includesa first sheet feed roller 31 a. The second sheet cassette 32 includes asecond sheet feed roller 32 a. In the first and second sheet cassettes31 and 32, a stack of recording media P is stored, and the sheet feedrollers 31 a and 32 a contact the top sheet of the stack of therecording media P. As the first sheet feed roller 31 a is rotated by adriving device in a counterclockwise direction, the top sheet of therecording media P in the first sheet feed cassette 31 is fed to a sheetpassage 33 extending vertically at the right side of the first andsecond sheet cassettes 31 and 32. As the second sheet feed roller 32 ais rotated by a driving device in a counterclockwise direction, the topsheet of the recording media P in the second sheet feed cassette 32 isfed to the sheet passage 33. A plurality of conveyor roller pairs 34 isdisposed in the sheet passage 33, and the recording medium P fed to thesheet passage is interposed between the conveyor roller pairs 34 anddelivered upward along the sheet passage 33.

Substantially at the end of the sheet passage 33, a pair of registrationrollers 35 is disposed. The pair of registration rollers 35 temporarilystops rotating, immediately after the recording medium P delivered fromthe conveyor pairs 34 is interposed therebetween. The pair ofregistration rollers 35 starts to rotate again to feed the recordingmedium P to a later-described secondary transfer nip in appropriatetiming.

Still referring to FIG. 1, a description is provided of a transfer unit40. The transfer unit 40 is disposed above the image forming units 1Y,1C, 1M, and 1K. The transfer unit 40 includes an intermediate transferbelt 41 serving as an intermediate transfer member formed into anendless loop and rotated in the counterclockwise direction. The transferunit 40 includes the intermediate transfer belt 41, a belt cleaningdevice 42, a first bracket 43, a second bracket 44, four primarytransfer rollers 45Y, 45C, 45M, and 45K, a secondary transfer auxiliaryroller 46, a drive roller 47, an auxiliary roller 48, a tension roller49, and so forth. The intermediate transfer belt 41 is entrained aroundthese rollers and rotated endlessly in the counterclockwise direction bythe drive roller 47.

The primary transfer rollers 45Y, 45C, 45M, and 45K constitute primarytransfer devices. The intermediate transfer belt 41 is interposedbetween the primary transfer rollers 45Y, 45C, 45M, and 45K, and thephotosensitive drums 3Y, 3C, 3M, and 3K, thereby forming primarytransfer nips between the intermediate transfer belt 41 and the primarytransfer rollers 45Y, 45C, 45M, and 45K. A transfer bias having apolarity (for example, a positive polarity) opposite that of toner isapplied to a back surface of the intermediate transfer belt 41 (innercircumferential surface of the looped belt). A power source connected tofour primary transfer rollers 45Y, 45C, 45M, and 45K is under constantcurrent control or constant voltage control. As the intermediatetransfer belt 41 passes through the primary transfer nips of yellow,cyan, magenta, and black, the toner images of yellow, cyan, magenta, andblack on the photosensitive drums 3Y, 3C, 3M, and 3K, respectively, aretransferred onto the intermediate transfer belt 41 such that they aresuperimposed one atop the other, thereby forming a composite toner imageon the intermediate transfer belt 41 in the primary transfer process.

As illustrated in FIG. 1, the secondary transfer auxiliary roller 46serving as a secondary transfer device is disposed inside the loopformed by the intermediate transfer belt 41, opposite a secondarytransfer roller 50 which is disposed outside the loop. The intermediatetransfer belt 41 is interposed between the secondary transfer auxiliaryroller 46 and the secondary transfer roller 50, thereby forming asecondary transfer nip. The pair of registration rollers 35 feeds therecording medium P to the secondary transfer nip in appropriate timingsuch that the recording medium P is aligned with the composite tonerimage formed on the intermediate transfer belt 41 in the secondarytransfer nip. The composite toner image is transferred secondarily ontothe recording medium P due to a secondary transfer electric fieldgenerated between the secondary transfer auxiliary roller 46 and thesecondary transfer roller 50 and a nip pressure applied to the secondarytransfer nip. Accordingly, the full-color toner image is formed on therecording medium P.

After the intermediate transfer belt 41 passes through the secondarytransfer nip, residual toner not having been transferred onto therecording medium P remains on the intermediate transfer belt 41. Theresidual toner is removed by the belt cleaning device 42. The beltcleaning device 42 includes a cleaning blade 42 a which contacts thesurface of the intermediate transfer belt 41 to remove the residualtoner therefrom.

Substantially above the secondary transfer nip in FIG. 1, a fixingdevice 60 for fixing the toner image on the recording medium P isdisposed. The fixing device 60 includes a heating-pressure roller 61 anda fixing belt assembly 62. The heating-pressure roller 61 includes abuilt-in heat source such as a halogen lamp. The fixing belt assembly 62includes a fixing belt 64 serving as a fixing device, a heating roller63 including a heat source 63 a such as a halogen lamp inside thereof, atension roller 65, and a drive roller 66. The fixing belt 64 isentrained around the heating roller 63, the tension roller 65, and thedrive roller 66, and is moved in the counterclockwise direction. Whilemoving endlessly, the fixing belt 64 is heated by the heating roller 63from the back. The heating-pressure roller 61 rotating in the clockwisedirection contacts the outer circumferential surface of the fixing belt64 wound around the heating roller 63, thereby forming a fixing nip atwhich the heating-pressure roller 61 contacts the fixing belt 64.

A temperature detector is disposed outside the loop formed by the fixingbelt 64 with a certain gap therebetween. The temperature detectordetects a surface temperature of the fixing belt 64 immediately beforethe fixing belt 64 enters the fixing nip. Detection results are sent toa power source circuit. Based on the detection results provided by thetemperature detector, the power source circuit controls electricalcontinuity between the power source and the heat source 63 a of theheating roller 63, and between the power source and the heat source 61 aof the heating-pressure roller 61. Accordingly, the surface temperatureof the fixing belt 64 is maintained approximately at 140° C., forexample.

After passing through the secondary transfer nip and separating from theintermediate transfer belt 41, the recording medium P is sent to thefixing device 60. In the fixing device 60, the composite toner image isfixed onto the recording medium P as the recording medium P is heatedand pressed by the fixing belt 64 and the heating-pressure roller 61while passing through the fixing nip upward.

The recording medium P after fixing is discharged outside the imageforming apparatus through a pair of sheet output rollers 67. A sheetstack portion 68 is formed on the upper surface of a main body of theimage forming apparatus. The recording medium P discharged outside theimage forming apparatus is stacked onto the sheet stack portion 68.

According to the present illustrative embodiment, an image is formedwith at least one arbitrarily chosen color. A description is nowprovided of an example of formation of an image in a black, single colormode (second control mode) and a full-color mode (first control mode).In the black single color mode, only the toner image formed on thephotosensitive drum 3K of the color black disposed at the extremedownstream end in the moving direction of the intermediate transfer belt41 is transferred primarily onto the intermediate transfer belt 41, andthen transferred secondarily onto a recording medium P, thereby forminga single color image (monochrome image) of the color black. In thefull-color mode, the toner images formed on all the photosensitive drums3Y, 3C, 3M, and 3K are transferred primarily onto the intermediatetransfer belt 41 such that they are superimposed one atop the other,forming a composite toner image. Subsequently, the composite toner imageis transferred secondarily onto a recording medium P, thereby forming afour-color (full-color image).

Four toner cartridges 100Y, 100C, 100M, and 100K storing toners ofyellow, cyan, magenta, and black, respectively, are disposed above thetransfer unit 40. The toner in the toner cartridges 100Y, 100C, 100M,and 100K is supplied to development devices 7Y, 7C, 7M, and 7K of theimage forming units 1Y, 1C, 1M, and 1K, respectively. The tonercartridges 100Y, 100C, 100M, and 100K are detachably attachable relativeto the main body of the image forming apparatus, independent of theimage forming units 1Y, 1C, 1M, and 1K.

As illustrated in FIG. 2, the photosensitive drum unit 2Y includes thephotosensitive drum 3Y, a drum cleaning device 4Y, a charge remover, acharging device 5Y for charging the surface of the photosensitive drum3Y, and so forth. The charging device 5Y charges uniformly the surfaceof the photosensitive drum 3Y rotated by a driving device in theclockwise direction indicated by an arrow in FIG. 2. In theconfiguration shown in FIG. 2, the charging device 5Y includes acharging roller 6Y to which a charging bias is applied by a powersource. The charging roller 6Y is rotated in the counterclockwisedirection and contacts or approaches the photosensitive drum 3Y tocharge uniformly the surface of the photosensitive drum 3Y.

Alternatively, a charging brush may be employed instead of the chargingroller 6Y and may directly contact the photosensitive drum 3Y, or ascorotron charger or the like may be employed to charge uniformly thephotosensitive drum 3Y. The uniformly charged surface of thephotosensitive drum 3Y is scanned by the laser light L projected fromthe optical writing unit 20, thereby forming an electrostatic latentimage for the color yellow on the surface of the photosensitive drum 3Y.

The developing unit 7Y includes a first chamber 9Y and a second chamber14Y. The first chamber 9Y includes a first conveyor screw 8Y. The secondchamber 14Y includes a toner density detector 10Y comprised of amagnetic permeability sensor or the like, a second conveyor screw 11Y, adevelopment roller 12Y as a developing agent bearing member, a doctorblade 13Y as a developing agent regulator, and so forth. The firstchamber 9Y and the second chamber 14Y store a yellow developing agentconsisting of negatively chargeable yellow toner particles and magneticcarriers. The first conveyor screw 8Y is driven to rotate by a drivesource and delivers the yellow developing agent in the first chamber 9Yfrom the proximal side to the distal side in a direction perpendicularto the drawing surface. The yellow developing agent is delivered to thesecond chamber 14Y through a communication hole formed in a wallseparating the first chamber 9Y and the second chamber 14Y.

The second conveyor screw 11Y in the second chamber 14Y is driven torotate and delivers the yellow developing agent from the proximal sideto the distal side. During the delivery of the yellow developing agent,the toner density detector 10Y fixed to the bottom of the first chamber9Y detects the density of toner. Substantially above the second conveyorscrew 11Y, the development roller 12Y is disposed parallel to the secondconveyor screw 11Y. The development roller 12Y comprises a developmentsleeve 15Y made of a non-magnetic pipe which is rotated in thecounterclockwise direction, and a magnetic roller 16Y disposed insidethe development sleeve 15Y. A portion of the developing agent deliveredby the second conveyor screw 11Y is carried onto the surface of thedevelopment sleeve 15Y by the magnetic force of the magnetic roller 16Y.

After the thickness of the developing agent layer on the developmentsleeve 15Y is regulated by the doctor blade 13Y which is spaced apart acertain distance from the development sleeve 15Y, the developing agentis delivered to a developing area facing the photosensitive drum 3Y andthe yellow toner is adhered to the electrostatic latent image on thephotosensitive drum 3Y, thereby forming a yellow toner image on thephotosensitive drum 3Y. The yellow developing agent consumed indevelopment is returned onto the second conveyor screw 11Y as thedevelopment sleeve 15Y rotates. When the yellow developing agent isdelivered to the proximal end in FIG. 2, the yellow developing agent isreturned to the first chamber 9Y through the communication opening.

The detection result of the magnetic permeability of the yellowdeveloping agent detected by the toner detector 10Y is provided as avoltage signal to a controller 200. In order to show a correlationbetween the toner density of the yellow developing agent and themagnetic permeability of the yellow developing agent, the toner densitydetector 10Y outputs a voltage corresponding to the yellow tonerdensity.

The controller 200 includes a storage device such as Random AccessMemory (RAM), developing agent condition detectors 200K, 200C, 200M, and200Y for detecting a degree of deterioration of each of the developingagents for the colors black, cyan, magenta, and yellow, respectively, aprimary transfer bias adjustor 200 a for adjusting a primary transferbias, and a secondary transfer bias adjustor 200 b for adjusting asecondary transfer adjustor. A target output voltage Vtref output fromthe toner density detector 10Y and other target output voltages Vtrefoutput from each of the toner density detectors for the colors cyan,magenta, and black are stored in the storage device of the controller200.

As for the development device 7Y, the output voltage provided by thetoner density detector 10Y is compared with the target output voltageVtref for the color yellow, and a later-described toner supply devicefor the yellow toner is driven in accordance with the result ofcomparison. Accordingly, an appropriate amount of yellow toner issupplied to the developing agent in the first chamber 9Y from which theyellow toner is consumed and the toner density of which has droppedduring development. The yellow toner density in the second chamber 14Yis maintained within a predetermined range. Similarly, the same tonersupply control is carried out for the developing agents of differentcolors, i.e., developing agents in the image forming units 1C, 1M, and1K.

The toner image of yellow formed on the photosensitive drum 3Y istransferred primarily onto the intermediate transfer belt 41. The drumcleaning device 4Y removes residual toner remaining on the surface ofthe photosensitive drum 3Y after the primary transfer process. Thecharge remover removes residual charge remaining on the photosensitivedrum 3Y after the surface thereof is cleaned by the drum cleaning device4Y so that the surface of the photosensitive drum 3Y is initialized inpreparation for the subsequent imaging cycle. Similarly, in the imageforming units 1C, 1M, and 1K, a cyan toner image, a magenta toner image,and a black toner image are formed on the photosensitive drums 3C, 3M,and 3K, respectively, and transferred primarily onto the intermediatetransfer belt 41.

The primary transfer rollers 45Y, 45C, 45Y, and 45K are made of a metalcore and a rubber material having a medium electrical resistivity woundaround the metal core. More specifically, the rubber material is foamrubber having a volume resistivity preferably in a range of from 10⁶Ω·cm to 10¹⁰ Ω·cm, more preferably, in a range of from 10⁷ Ω·cm to 10⁹Ω·cm. The rubber material is not limited to a foam rubber.Alternatively, it may be solid rubber having a medium electricalresistivity.

The secondary transfer auxiliary roller 46 is made of a metal core and arubber material having a medium electrical resistivity wound around themetal core. More specifically, the rubber material is solid rubberhaving a medium electrical resistivity and a volume resistivitypreferably in a range of from 10⁶ Ω·cm to 10¹⁰ Ω·cm, more preferably, ina range of from 10⁷ Ω·cm to 10⁹ Ω·cm.

The secondary transfer roller 50 may be formed of foam rubber having amedium electrical resistivity. The volume resistivity thereof ispreferably in a range of from 10⁶ Ω·cm to 10¹⁰ Ω·cm, and morepreferably, in a range of from 10⁷ Ω·cm to 10⁹ Ω·cm.

The intermediate transfer belt 41 is a three-layer belt including a baselayer, an elastic layer, and a surface layer. The base layer has athickness in a range of from 50 μm to 100 μm and is formed of resinhaving a medium electrical resistivity such as polyimide (PI),polyamide-imide (PAD, polycarbonate (PC), ethylene tetrafluoroethylene(ETFE), polyvinylidene fluoride (PVDF), polyphenylene sulfide (PPS), andthe like, the resistivity of which is adjusted by dispersing carbon orion conductor.

The elastic layer is disposed on the base layer and has a thickness in arange of from 100 μm to 500 μm. The elastic layer is formed of a rubbermaterial such as urethane, nitrile butadiene (NBR), chloroprene (CR),and the like, the resistivity of which is adjusted similarly bydispersing carbon or ion conductor. The surface layer is formed offluoro-rubber or resin (or a hybrid material consisting of thesematerials), having a thickness in a range of from 1 μm to 10 μm, and isdisposed on the elastic layer.

More specifically, the volume resistivity of the intermediate transferbelt 41 is preferably in a range of from 10⁶ Ω·cm to 10¹⁰ Ω·cm, morepreferably, in a range of from 10⁸ Ω·cm to 10¹⁰ Ω·cm. The surfaceresistivity of the intermediate transfer belt 41 is preferably in arange of from 10⁶ Ω/sq to 10¹² Ω/sq, more preferably, in a range of from10⁸ Ω/sq to 10¹² Ω/sq. Furthermore, Young's modulus (modulus oflongitudinal elasticity) of the base layer is preferably 3000 Mpa ormore so that adequate mechanical strength to resist stretching, bending,creasing, and waving is obtained. With the use of such an elasticintermediate transfer belt, transferability of toner to a recordingmedium having a low paper fiber density and paper with a coarse surfacesuch as embossed paper having an embossed groove depth of approximately20 μm to 30 μm is enhanced because the elastic layer conforms to theshape of the recessed portion on the surface of the recording medium,thereby reliably transferring the toner to the recording medium.

As another example of the intermediate transfer belt, a single-layerbelt may be used. Such a single-layer belt includes a resin layer havinga medium electrical resistivity. The material of the belt includes, butis not limited to, polyimide (PI), polyamide-imide (PAI), polycarbonate(PC), ethylene tetrafluoroethylene (ETFE), polyvinylidene fluoride(PVDF), polyphenylene sulfide (PPS), and the like, the resistivity ofwhich is adjusted by dispersing carbon or ion conductor. Alternatively,the single-layer belt thus obtained may be provided with a top layerhaving a high electrical resistivity which is slightly higher than thevolume resistivity of the belt layer. The thickness of the top layer ispreferably in a range of from 1 μm to 10 μm.

This type of belt controls the resistivity by dispersing carbon in theresin and is known to suppress a so-called “white void” generated on arecording medium during secondary transfer. More specifically, themoisture content of the recording medium decreases after fixing, whichcauses an increase in the resistivity of the recording medium. When therecording medium with the increased resistivity undergoes the secondarytransfer process, white voids are generated on the recording medium.White voids refer to dropouts of toner which occur in a path throughwhich the transfer current flows in a concentrated manner due to unevendispersion of carbon in the belt. Consequently, the toner in the path isrepelled, and hence the output image has white voids at the placecorresponding to the path. With the high-resistivity layer provided tothe surface of the belt, the local concentration of the transfer currentis reduced, thereby reducing the white spots.

Next, a description is provided of relations between the primarytransfer current and the primary transfer rate according to a firstillustrative embodiment.

In the first illustrative embodiment, a power source connected to theprimary transfer rollers 45Y, 45C, 45M, and 45K is under constantcurrent control so as to maintain a target current value (i.e., primarytransfer current set value). In the constant current control, as shownin FIGS. 12 and 13, the relations between the primary transfer currentand the primary transfer rate change depending on an image area ratio inthe main scanning direction.

A non-image portion on the photosensitive drums 3Y, 3C, 3M, and 3K, atwhich no laser light L is illuminated, has a relatively large amount ofelectrical charge having the same polarity (in the present illustrativeembodiment, a negative polarity) as that of toner. Thus, the primarytransfer current flowing upon application of the primary transfer biasis used to attract the negative-charged toner and to attract thenegative charge at the non-image portion. In general, the negativecharge at the non-image portion has more electrical charge per unit areathan the toner. Thus, with an increase in the area of the non-imageportion, a greater amount of the primary transfer current needs to flowto transfer the same amount of toner to the intermediate transfer beltside. Therefore, as illustrated in FIGS. 12 and 13, a peak primarytransfer current capable of obtaining the maximum primary transfer rateshifts to a higher primary transfer current side for a 5% band imagethan a 95% band image.

Similarly, deterioration of the developing agent with time and hence adecrease in the charge amount of toner have a pronounced influence onformation of an image with a high image area ratio in the main scanningdirection. More specifically, as illustrated in FIG. 13, for the 95%band image using a deteriorated developing agent with time, the peakprimary transfer current capable of achieving the maximum primarytransfer rate shifts to the lower primary transfer current side.Therefore, when the charge amount of overall toner drops due todeterioration of the developing agent with time, the primary transfercurrent (peak) for achieving the maximum primary transfer rate in the95% band image and the primary transfer current (peak) for achieving themaximum primary transfer rate in the 5% band image depart from eachother. At this time, if an initial set value for the primary transfercurrent (optimum value shown in FIG. 12) is applied even after thedeveloping agent deteriorated with time, the primary transfer rate inthe 95% band image drops significantly and degradation of image qualitybecomes more pronounced. In particular, degradation of image quality inwhich an image density fluctuates significantly in accordance with theimage area ratio of the output image becomes more significant.Therefore, it is desirable to correct the primary transfer current inaccordance with the degree of deterioration of the developing agent.

Next, a description is provided of a secondary transfer rate.

FIG. 3 is a graph showing relations of a secondary transfer current anda secondary transfer rate for a black single color toner image, a cyansingle color toner image, and a two-color toner image with cyan andmagenta when the primary transfer current is changed for the black tonerimage.

FIG. 3 shows a primary transfer current set value T1 being set to 20 μA(T1=20) and 40 μA (T1=40) for the photosensitive drum 3K disposed at theextreme downstream end in the moving direction of the intermediatetransfer belt 41.

With respect to the black single color toner image, there is asignificant difference in the relations of the secondary transfercurrent and the secondary transfer rate when the primary transfercurrent set value T1 is 20 μA and when the primary transfer current setvalue T1 is 40 μA. More specifically, when the primary transfer setvalue T1 for the color black is set to a higher value, i.e., 40 μA(T1=40), the secondary transfer current (peak) capable of achieving themaximum secondary transfer rate is at a higher secondary transfercurrent side. By contrast, when the primary transfer set value T1 forblack is set to a lower value, i.e., 20 μA (T1=20), the secondarytransfer current (peak) capable of achieving the maximum secondarytransfer rate is at a lower secondary transfer current side.

With respect to the cyan single color toner image and the two-colortoner image with cyan and magenta, there is no significant difference inthe relations of the secondary transfer current and the secondarytransfer rate when the primary transfer current set value T1 is 20 μAand when the primary transfer current set value T1 is 40 μA. Althoughthe above example relates to the cyan single color toner image, the sameor the similar result as the cyan single color image is expected for themagenta single color toner image and the yellow single color tonerimage.

With respect to the two-color toner image with cyan and magenta, thesecondary transfer current (peak) capable of achieving the maximumsecondary transfer rate is at a higher secondary transfer current sidethan the single color toner images such as the black single color tonerimage and the cyan single color toner image. The reason is as follows.Although the above example relates to the two-color toner image withcyan and magenta, the same or the similar result as the two-color tonerimage with cyan and magenta is expected for other color combinationsconsisting of two or more colors.

FIG. 4 a graph showing relations of the primary transfer current for theblack toner image and the charge amount of toner for each toner imagebefore secondary transfer.

With respect to the black toner image, as the primary transfer currentfor black is increased, the charge amount of toner (charge amount oftoner before secondary transfer) on the intermediate transfer belt 41increases at a large rate. By contrast, with respect to the cyan tonerimage, as the primary transfer current for black is increased, thecharge amount of toner (charge amount of toner before secondarytransfer) on the intermediate transfer belt 41 increases, but the rateof increase is less than the that of the black toner image. With respectto the magenta toner image and the cyan toner image, the charge amountof toner (charge amount of toner before secondary transfer) on theintermediate transfer belt 41 hardly changes relative to the changes inthe primary transfer current for black.

The black toner image is formed on the photosensitive drum 3K disposedat the extreme downstream end in the moving direction of theintermediate transfer belt 41. Thus, toner not having been charged up bythe primary transfer current gets charged up for the first time by theprimary transfer current for the color black. As a result, the rate ofincrease in the charge amount of toner (charge amount of toner beforesecondary transfer) relative to the change in the primary transfercurrent for the color black is large.

By contrast, the cyan toner image is formed on the photosensitive drum3C which is the second photosensitive drum from the extreme downstreamend in the moving direction of the intermediate transfer belt 41. Thus,after the cyan toner image is charged by the primary transfer currentduring its own primary transfer, the cyan toner image is charged up bythe primary transfer current for the color black. As a result, by thetime the cyan toner image is charged up by the primary transfer currentfor the color black, the charge amount of toner has been increased to acertain level and the rate of increase in the charge amount (chargeamount of toner before secondary transfer) of the cyan toner relative tothe change in the primary transfer current for the color black is low.

Furthermore, the magenta toner image and the yellow toner image areformed on the photosensitive drums 3M and 3Y which are the third andfourth photosensitive drums from the extreme downstream end in themoving direction of the intermediate transfer belt 41, respectively.Thus, these toner images have been charged at least twice by the primarytransfer current by the time these toner images are charged up by theprimary transfer current for the color black. As a result, by the timethese toner images are charged up by the primary transfer current forthe color black, the charge amount of toner has been increased to alevel substantially near the saturation level, and hence the rate ofincrease in the charge amount (charge amount of toner before secondarytransfer) of the magenta and cyan toner is not influenced.

The higher is the charge amount of toner in the secondary transferportion, that is, the higher is the charge amount of toner prior tosecondary transfer, the more secondary transfer current is needed. Thus,as illustrated in FIG. 3, the peak value of the secondary transfercurrent for the cyan single color toner image capable of achieving themaximum secondary transfer rate is at a higher secondary transfercurrent side than that of the black single color toner image. The peakvalue of the secondary transfer current for the two-color toner imagewith cyan and magenta capable of achieving the maximum secondarytransfer rate is at a higher secondary transfer current side than thatof the cyan single color toner image.

Due to such tendencies, the value of the secondary transfer current inthe full-color mode is set to achieve a highest possible secondarytransfer rate within a range in which the secondary transfer rates forthe plurality of toner images constituting the composite toner image aresimilar or substantially the same (that is, none of the secondarytransfer rates has a low value relative to all the other ratios). Morespecifically, according to the present illustrative embodiment, as willbe described later, the primary transfer bias adjuster 200 a of thecontroller 200 adjusts, or more specifically reduces the primarytransfer current for the color black with time (from T1=40 to T1=20).Accordingly, the set value for the secondary transfer current in thefull-color mode is determined, for example, as similar to or the samevalues as shown in FIG. 3 in consideration of the secondary transferrates for the black single color toner image, the cyan single colortoner image, and the two-color toner image before and after thecorrection (in a case of T1=40 and T1=20).

By contrast, the set value for the secondary transfer current in theblack single color mode (monochrome mode) needs to take only thesecondary transfer rate for the black single color toner image intoconsideration. Therefore, the set value for the secondary transfercurrent in the black single color mode is determined to be similar orthe same values as shown in FIG. 3 to achieve the highest possiblesecondary transfer rate within a range in which substantially the samesecondary transfer rate can be achieved in consideration of thesecondary transfer rate for the black toner image before and after thecorrection of the primary transfer current for the color black (in acase of T1=40 and T1=20).

According to the first illustrative embodiment of the presentdisclosure, the primary transfer current for the color black is adjustedto decrease with time (from T1=40 to T1=20). Accordingly, the secondarytransfer current for achieving the maximum secondary transfer rate forthe black single color toner image after the correction (in a case ofT1=20) shifts towards the lower secondary transfer current side. At thistime, in the single color mode for forming a single color toner image ofblack, the set value for the secondary transfer current is set so as notto change before and after the correction as described above. Therefore,in the black single color mode, even when the primary transfer currentfor the color black is adjusted to decrease, the secondary transfer ratefor the black single color toner image formed in the black single colormode does not change. Accordingly, in the black single color mode, theprimary transfer current for the color black is reduced to preventdegradation of image quality such as a decrease in the image densityattributed to a decrease in the primary transfer rate due to a reducedoverall charge amount of toner caused by the deterioration of thedeveloping agent.

However, if the primary transfer current for the color black is reducedin the full-color mode in the similar manner as in the black singlecolor mode, there is a possibility that the degradation of image qualityis not prevented, but rather may be worsened.

As described above, in the full-color mode, the secondary transfercurrent is set as shown in FIG. 3. With such a set value for thesecondary transfer current shown in FIG. 3, reducing the primarytransfer current for the color black with time (from T1=40 to T1=20)causes the secondary transfer rate for the black single color tonerimage after correction (when T1=20) to drop significantly from thesecondary transfer rate before the correction. Consequently, in thefull-color mode, reducing the primary transfer current for the colorblack in accordance with the degree of deterioration of the developingagent causes the secondary transfer rate for the black toner imageformed in the full-color mode to drop significantly as compared with thesecondary transfer rate thereof before the adjustment, hence causingdegradation of image quality such as a reduced image density of thecolor black and a change in the color balance relative to other colors.Thus, if the primary transfer current for the color black is reduced inthe full-color mode in the similar manner as in the black single colormode, there is a possibility that the degradation of image quality isnot prevented, but rather may be worsened.

In view of the above, according to the first illustrative embodiment ofthe present disclosure, the degree of deterioration of the blackdeveloping agent is detected by the developing agent condition detector200K and the primary transfer current for the color black is adjusted bya certain amount corresponding to the degree of deterioration in theblack single color mode. However, in the full-color mode, such anadjustment of the primary transfer current for the color black is notperformed, or the primary transfer current for the color black isadjusted by a smaller correction amount than the black single colormode.

Furthermore, according to the first illustrative embodiment, the degreeof deterioration of other developing agents such as for the colors cyan,magenta, and yellow, other than the black developing agent is detectedindividually by the developing agent condition detectors 200C, 200M, and200Y and the set value for each primary transfer current is adjusted bya certain amount corresponding to the deterioration of the respectivedeveloping agent. The primary transfer current adjustment may beperformed in the black single color mode and in the full-color mode.However, in a case in which the same failure as in the adjustment of theprimary transfer current for the color black occurs, in the full-colormode, such an adjustment of the primary transfer current is notperformed for the second color, i.e., cyan, or in some cases the thirdcolor, i.e., magenta from the extreme downstream end in the movingdirection of the intermediate transfer belt 41, or the primary transfercurrent for cyan or magenta is adjusted by a smaller correction amountthan the respective single color mode.

According to the first illustrative embodiment, the set value for thesecondary transfer current may be set unchangeable in accordance withthe degree of deterioration of the developing agent. Alternatively, theset value for the secondary transfer current may be changed inaccordance with other conditions. Preferably, the secondary transferbias adjustor 200 b adjusts the set value for the secondary transfercurrent in accordance with the degree of deterioration of the blackdeveloping agent in the image forming unit 1K disposed at the extremedownstream end in the moving direction of the intermediate transfer belt41 or in accordance with the adjustment of the set value for the primarytransfer current for the color black. In this case, preferably, theamount of adjustment (correction) of the secondary transfer current isgreater in the black single color mode than in the full-color mode.

FIG. 5 is a flowchart showing steps in a process of determination of anamount of adjustment (correction) in accordance with a degree ofdeterioration of a developing agent.

FIG. 6 is a table showing an example of set values of the primarytransfer current for each color in accordance with the degree ofdeterioration of the developing agent and set values of the secondarytransfer current.

In the first illustrative embodiment, in order to determine the degreeof deterioration of the developing agent of each color, the degree ofdeterioration of the developing agent is compared with a presetthreshold value. In the first illustrative embodiment, the degree ofdeterioration of the developing agent is classified into three groups:“INITIAL STATE”, “ELAPSED TIME 1”, and “ELAPSED TIME 2”.

More specifically, as illustrated in FIG. 5, the degree of deteriorationof the developing agent is calculated at step S1 (S1). In the firstillustrative embodiment, a degree of deterioration of the developingagent refers to deterioration of the developing agent in terms ofchargeability of toner. Because direct observation of such a degree ofdeterioration of the developing agent is difficult, parameters that arein correlation with the degree of deterioration of the developing agentare used to detect the degree of deterioration of the developing agentin directly.

The parameters to obtain the degree of deterioration of the developingagent include, but are not limited to a number of passing sheets, atravel distance of the development roller 12 (a drive distance of thesurface of the development roller 12 in the circumferential direction),a total amount of consumption of toner consumed in the development unit7 detected by the toner density detector 10Y, and an elapsed time from atime at which a new developing agent is set to the development unit. Atleast one parameter or a combination of two or more parameters that arein correlation with the degree of deterioration of the developing agentis used. A gradient (development y) of an amount of development relativeto the development bias or the like upon adjustment of image quality(upon process control) is in correlation with the degree ofdeterioration of the developing agent and hence can be used as aparameter to obtain the degree of deterioration of the developing agent.

In a configuration in which replacement of parts in the development unit7 and the photosensitive drum unit 2 is carried out together as a set,chronological change characteristics of the photosensitive drum (traveldistance of the photosensitive drum 3) is in correlation with the degreeof deterioration of the developing agent and hence can be used as aparameter to obtain the degree of deterioration of the developing agent.According to the present illustrative embodiment, a value obtained inthe following equation is used as an example of the degree ofdeterioration of the developing agent.DEGREE OF DETERIORATION OF DEVELOPING AGENT=NUMBER OF SHEETSPASSED[kp]×TOTAL AMOUNT OF TONER CONSUMPTION[kg]  [EQUATION 1]

With a large number of passing sheets, the development unit 7 operateslonger and the developing agent in the development unit 7 is mixed bythe first conveyor screw 8Y and the second conveyor screw 11Y, therebyincreasing the degree of deterioration of the developing agent. Inparticular, the more is the developing agent that is mixed, the moretoner particles separate from an additive, causing the additive to stickto the surface of carrier particles, thereby reducing the chargeabilityof the toner. As a result, the degree of deterioration of the developingagent increases.

It is known that the degree of deterioration of the developing agent isin high correlation with the total amount of toner consumption. That is,the degree of deterioration of the developing agent increasesproportionally with the total amount of toner consumption. Therefore,not only the total number of sheets passing, but also the total amountof toner consumption is used as a parameter for calculation of thedegree of deterioration of the developing agent.

In the first illustrative embodiment, a two-component developing agentconsisting of toner and carrier is employed. Thus, the cause of thedeterioration of the developing agent can be classified roughly into twocategories. One is a decrease in chargeability of toner attributed todeterioration of the toner itself, and another is a decrease inchargeability of toner attributed to the carrier. In the firstillustrative embodiment, the degree of deterioration of the developingagent is detected by focusing on the decrease in the chargeability oftoner attributed to the carrier. When the amount of toner consumedduring printing of a certain number of sheets is equal to or less than aspecified amount, the toner in the development unit is dischargedforcibly to the photosensitive drum 3, thereby replacing the old tonerin the development unit 7 with the new toner in toner replacementcontrol. Therefore, the deterioration of toner itself does not affectthe decrease in the charge amount of toner much. In view of this, in thefirst illustrative embodiment, the degree of deterioration of thedeveloping agent is detected by focusing on the decrease in thechargeability of toner attributed to the carrier.

Referring back to FIG. 5, after calculation of the degree ofdeterioration of the developing agent, next, whether the degree ofdeterioration of the developing agent is less than a threshold value 1is determined at step S2. When the degree of deterioration is determinedto be less than the threshold value 1 (YES, S2), the degree ofdeterioration of the developing agent is classified as “INITIAL STATE”at step S3 (S3). When the degree of deterioration is equal to or greaterthan the threshold value 1 (NO, S2), whether the degree of deteriorationof the developing agent is less than a threshold value 2 is determinedat step S4 (S4). When the degree of deterioration is determined to beless than the threshold value 2 (YES, S4), the degree of deteriorationof the developing agent is classified as “ELAPSED TIME 1” at step S5(S5). When the degree of deterioration is equal to or greater than thethreshold value 2 (NO, S4), the degree of deterioration of thedeveloping agent is classified as “ELAPSED TIME 2” at step S6 (S6). Thethreshold value 1 and the threshold value 2 for determination of thedegree of deterioration of the developing agent are, for example,10[kp×kg] and 50 [kp×kg], respectively.

In this configuration, the degree of deterioration of the developingagent for each of the colors yellow, magenta, cyan, and black isclassified independently into three categories: INITIAL, ELAPSED TIME 1,and ELAPSED TIME 2.

In the first illustrative embodiment, a table illustrated in FIG. 6showing the relations between the degree of deterioration of thedeveloping agent (time group), the set values for the primary transfercurrent for each color in each mode, and the set value for the secondarytransfer current is stored in the storage device in advance. Thecontroller 200 of the first illustrative embodiment enables applicationof the primary transfer current and the secondary transfer current tomeet the respective set values for the primary transfer current and thesecondary transfer current specified by the table in accordance with thecalculated degree of deterioration of the developing agent (time group)as described above in accordance with the control mode upon imageforming operation.

In the first illustrative embodiment, as illustrated in FIG. 6, thecorrection amount of the primary transfer current of the image formingunit 1K for the color black disposed at the extreme downstream end inthe moving direction of the intermediate transfer belt 41 in accordancewith the degree of deterioration of the developing agent (time group) inthe black single color mode is less than that in the full-color mode.With this configuration, in the black single color mode in which theinfluence of the change (correction) in the set value of the primarytransfer current on the secondary transfer rate is relatively small, thedegradation of image quality attributed to the decrease in the primarytransfer rate associated with the deterioration of the developing agentcan be suppressed properly. Furthermore, in the full-color mode in whichthe influence of the change (correction) in the set value of the primarytransfer current on the secondary transfer rate for the color black issignificant, by reducing the amount of correction of the primarytransfer current the influence on the secondary transfer rate can besuppressed and the degradation of overall image quality can beprevented.

The principle of the amount of correction of the secondary transfercurrent is the same that of the primary transfer current.AMOUNT OF CORRECTION=(INITIAL VALUE−VALUE AFTER CORRECTION)/INITIALVALUE  [EQUATION 2]

According to the present illustrative embodiment, in the full-colormode, the amount of correction of the primary transfer current for thecolor black in accordance with the degree of deterioration of thedeveloping agent (time group) is less than in the black single colormode. Alternatively, however, as illustrated in FIG. 7, in thefull-color mode, the primary transfer current for the color black doesnot have to be corrected in accordance with degree of deterioration ofthe developing agent (time group). In this case, in the full-color mode,there is no need to change (adjust) the setting of the primary transfercurrent for the color black in accordance with the degree ofdeterioration of the developing agent (time group), thereby allowingsimplification of the control.

Furthermore, as illustrated in FIG. 6, the amount of correction of thesecondary transfer current in accordance with the degree ofdeterioration of the developing agent (time group) is greater in theblack single color mode than in the full-color mode. As illustrated inFIG. 3, the secondary transfer current (peak) capable of achieving themaximum secondary transfer rate for the two-color toner image with cyanand magenta is hardly affected even when the primary transfer currentfor the color black is changed. The same result is obtained when theprimary transfer current for other colors is changed. Because the sameresult as the two-color toner image is obtained for the four-color tonerimage, in the full-color mode there is less need to adjust the secondarytransfer current in accordance with the adjustment of the primarytransfer current. However, since the secondary transfer rate for theblack toner image drops significantly due to adjustment of the primarytransfer current, it is preferable to make some adjustments in thefull-color mode.

In the black single color mode, as illustrated in FIG. 3, the adjustmentof the primary transfer current causes the secondary transfer current(peak) for achieving the maximum secondary transfer rate for the blacksingle color toner image to shift towards the lower secondary transfercurrent side. Thus, as in the first illustrative embodiment, whensetting the secondary transfer current to achieve the maximum secondarytransfer rate in the initial state, Elapsed Time 1, and Elapsed Time 2,the amount of change (correction) in the setting is greater in the blacksingle color mode than in the full-color mode.

Furthermore, in the first illustrative embodiment, the amount ofadjustment (correction) of the primary transfer current in the imageforming units 1Y, 1M, and 1C for the colors yellow, magenta, and cyan,respectively, disposed upstream from the image forming unit 1K for thecolor black which is disposed at the extreme downstream end in themoving direction of the intermediate transfer belt 41 is greater thanthe amount of adjustment (correction) of the primary transfer currentfor the color black in the image forming unit 1K. The reason is asfollows.

The toner images primarily transferred from the image forming units 1Y,1M, and 1C disposed upstream from the image forming unit 1K are chargedup by the primary transfer current of the downstream image forming unit1K, and advance to the secondary transfer portion. Consequently, evenwhen the primary transfer current of the image forming units 1Y, 1M, and1C is changed upon primary transfer, the charge amount of toner of theprimarily transferred toner images is less affected so that thesecondary transfer rate is less affected. Therefore, the primarytransfer current of the image forming units 1Y, 1M, and 1C in theupstream side is adjusted while taking into consideration of onlyimproving the primary transfer rate which decreases in accordance withthe degree of deterioration of the developing agent for each color.

By contrast, when the primary transfer current for the color black ischanged upon primary transfer, the charge amount of toner of theprimarily transferred black toner image drops significantly when theblack toner image arrives at the secondary transfer portion, therebyaffecting significantly the secondary transfer rate. Therefore, theprimary transfer current of the image forming unit 1K at the extremedownstream end needs to be adjusted (corrected) while taking intoconsideration of both enhancement of the primary transfer rate as wellas the decrease in the secondary transfer rate for the toner image ofthe color black. In view of the above, in the first illustrativeembodiment, in the full-color mode, the primary transfer current for thecolors yellow, magenta, and cyan is greater than that for the colorblack.

Next, a description is provided of a variation of adjustment control ofthe secondary transfer current in the first illustrative embodiment.

In the present variation, in accordance with an image area ratio of eachof the toner images, a set value of the secondary transfer current ischanged.

In the first illustrative embodiment, the secondary transfer bias isunder constant current control. Thus, similar to the relations betweenthe primary transfer current and the primary transfer rate, therelations between the secondary transfer current and the secondarytransfer rate change depending on the image area ratio. That is, thesecondary transfer current capable of achieving the optimum secondarytransfer rate depends on the image area ratio.

In view of the above, according to the present illustrative embodiment,the set value of the secondary transfer current is adjusted (corrected)in accordance with the image area ratio. However, as can be understoodfrom FIG. 3, in the full-color mode, the secondary transfer current(peak) capable of achieving the maximum secondary transfer rate for thetoner image of the color black and the secondary transfer current (peak)capable of achieving the maximum secondary transfer rate for the tonerimages of the colors yellow, cyan, and magenta separate from each otherdue to charging up by the primary transfer current. According to thepresent illustrative embodiment, in order to accommodate such separationof peaks, the adjustment control of the secondary transfer current inaccordance with the image area ratio is changed in accordance with thedegree of deterioration of the developing agent in the full-color mode.It is to be noted that in the black single color mode, the adjustmentcontrol of the secondary transfer current in accordance with the imagearea ratio is not changed in accordance with the degree of deteriorationof the developing agent.

FIG. 8 is a graph showing an example of a change in the secondarytransfer current setting according to the variation.

In general, the greater is the amount of toner that is present in thesecondary transfer portion, the more secondary transfer current isneeded. In view of the above, in the variation, first, an image arearatio of an image formed on a sheet of recording medium is obtained foreach of the colors yellow, magenta, and cyan. The image area ratios thusobtained are summed and the result is referred to as an image area ratioof the image (hereinafter referred to as total image area ratio). Theimage area ratio for each of the colors is a ratio of a portion to whichthe toner image of the respective color adheres relative to the entireimage area in the image on the sheet of recording medium. When the tonerimage of the respective color adheres to the entire image area, theimage area ratio of the respective color is 100%. Thus, in thevariation, when the image area ratio of each of the colors yellow,magenta, and cyan is 100%, the total image area ratio is the maximum300%.

In the variation, after a controller which performs imaging processingperforms color separation processing, the image area ratios of each ofthe colors yellow, magenta, and cyan are calculated and summed, therebyobtaining the total image area ratio. In accordance with the obtainedtotal image area ratio, the secondary transfer current is set inaccordance with the control shown in FIG. 8.

A broken line in FIG. 8 shows relations between the image area ratio andthe set value of the secondary transfer current in the initial state inwhich the degree of deterioration (time group) of the developing agentis at an initial phase. A solid line in FIG. 8 shows relations betweenthe image area ratio and the set value of the secondary transfer currentwhen the degree of deterioration (time group) of the developing agent isat Elapsed Time 2. Although not shown, when the degree of deterioration(time group) of the developing agent is at Elapsed Time 1, an average ofthe initial state and Elapsed time 2 becomes the set value of thesecondary transfer current.

As shown in FIG. 8, in the initial state in which the developing agenthas not deteriorated, the secondary transfer current is set to beproportional to the total image area ratio. By contrast, after an elapseof time, that is, after the developing agent deteriorated, as shown inFIG. 8, the secondary transfer current is adjusted such that thegradient of the secondary transfer current relative to the total imagearea ratio is relatively small, in particular, when the total image arearatio is low. More specifically, when the total image area ratio is 200%or more, it can be assumed that the substantially entire image area iscovered with toner of yellow, cyan, and magenta. Thus, after an elapseof time, simply the secondary transfer current in accordance with thetotal amount of toner to enter the secondary transfer portion issupplied without considering the black toner, which is the same controlas in the initial state.

By contrast, when the total image area ratio is less than 200%, thesecondary transfer current is controlled such that the higher is thedegree of deterioration of the developing agent, the lower is thesecondary transfer current. In this configuration, even when thesecondary transfer rate of the toner images of the colors yellow,magenta, and cyan is decreased to some extent, the secondary transfercurrent is prevented from being supplied excessively for the black tonerand hence good image quality is obtained entirely. At this time, in thearea with the total image area ratio of nearly 200%, the toner images ofthe colors yellow, magenta, and cyan have a high image area ratio sothat the secondary transfer current is controlled to rise quickly.However, in the area with a lower total image area ratio, the tonerimages for the colors yellow, magenta, and cyan do not need thesecondary transfer current much. Thus, the secondary transfer currentthereof has almost no gradient.

With this control, in the event of forming an image with a relativelylow image area ratio such as an image including a toner image of thecolor black in the full-color mode, a relatively low secondary transfercurrent is used in second transfer to obtain good image quality for thecolor black. By contrast, in an image with a relatively high total imagearea ratio, the colors yellow, magenta, and cyan are dominant. Thus,higher priority is given to the secondary transfer rate for the colorsyellow, magenta, and cyan than the color black, thereby obtaining goodimage quality for the colors yellow, magenta, and cyan.

In the present variation, the total image area ratio relative to theentire image area on a sheet of recording medium is used. Alternatively,an image area on a sheet of recording medium is segmented into aplurality of areas in the direction of conveyance of the recordingmedium (sub-scanning direction), and the secondary transfer current tobe supplied to each area that is present in the secondary transferportion is controlled individually based on the total image area ratioof each segmented area.

Next, with reference to FIGS. 9 through 11, a description is provided ofevaluation tests to evaluate effectiveness of the illustrativeembodiment of the present disclosure.

In the evaluation tests performed by the present inventors, the imagedensity was evaluated on all-solid single color images (image area ratioof 100%) of the colors black and cyan, a two-color solid image with cyanand magenta, and small toner patches having a size of 15 mm×15 mm forthe colors black and cyan, and a two-color toner patch of cyan andmagenta, each of which was formed on a sheet of recording medium.

In the first illustrative embodiment, as described above, both the setvalue of the primary transfer current and the set value of the secondarytransfer current are adjusted or corrected in accordance with the degreeof deterioration (time group) of the developing agent.

In a second illustrative embodiment using an image forming apparatushaving the same configuration as shown in FIG. 1, as described in thevariation, both the set value of the primary transfer current and theset value of the secondary transfer current are adjusted or corrected inaccordance with the degree of deterioration (time group) of thedeveloping agent, and further the secondary transfer current in thefull-color mode is set in accordance with the total image area ratio.

In a third illustrative embodiment using an image forming apparatushaving the same configuration as shown in FIG. 1, the set value of theprimary transfer current is adjusted or corrected in accordance with thedegree of deterioration (time group) of the developing agent similar tothe first illustrative embodiment, but the set value of the secondarytransfer current employed in the initial state remains unchanged evenafter an elapse of time.

In a comparative example 1 using an image forming apparatus having thesame configuration as shown in FIG. 1, the set value of the primarytransfer current and the set value of the secondary transfer currentused in the initial state remain unchanged even after an elapse of time.

In a comparative example 2 using an image forming apparatus having thesame configuration as shown in FIG. 1, the set value of the primarytransfer current used in the initial state remains unchanged, and theset value of the secondary transfer current is adjusted or corrected inaccordance with the degree of deterioration (time group) of thedeveloping agent such as in the first illustrative embodiment.

FIG. 9 is a table showing results of evaluation of the image densitywhen the degree of deterioration of the developing agent is in theinitial state.

FIG. 10 is a table showing results of evaluation of the image densitywhen the degree of deterioration of the developing agent is in ELAPSEDTIME 1.

FIG. 11 is a table showing results of evaluation of the image densitywhen the degree of deterioration of the developing agent is in ELAPSEDTIME 2.

In the comparative examples 1 and 2 in which the primary transfercurrent was not adjusted or corrected in accordance with the degree ofdeterioration of the developing agent, the primary transfer rate for theall-solid single color images of the colors black and cyan dropped, andthe image density thereof dropped significantly from the initial state.The image density was evaluated as POOR.

By contrast, in the first and second illustrative embodiments, the sameimage density as in the initial state was maintained for the all-solidsingle color images as well as the small toner patches of the colorsblack and cyan, the two-color solid image, and the two-color small tonerpatch even after an elapse of time.

In the third illustrative embodiment in which the secondary transfercurrent was not adjusted or corrected in accordance with the degree ofdeterioration of the developing agent, the secondary transfer rate forthe two-color solid image and the small toner patch of the color blackdropped, and the image density thereof dropped slightly from the initialstate after an elapse of time. However, the image density afterdegradation was still acceptable. Thus, the image density was evaluatedas FAIR.

Suitable toner for use in the above-described image forming apparatusesaccording to exemplary embodiments is described in detail below.

In order to obtain desirable charge on toner through charging in thedevelopment unit and through charging-up in the transfer portion, it wasassumed that using the toner having a volume resistivity greater than10.7 log Ωcm was preferable. However, in recent years, there is tonerhaving the volume resistivity of 10.7 log Ωcm or less in order to securefixation ability, in particular, at low temperature. Thus, there isdemand for an image forming apparatus capable of providing good imagequality using such toner. The toner having a relatively low volumeresistivity has a characteristic in that this type of toner is difficultto keep the charge. It is assumed that an apparent electrostaticcapacity decreases when electrical resistance decreases. Consequently,as the toner charging ability of carrier decreases with time, the chargeamount of toner having a low volume resistivity drops easily as comparedwith toner having a high volume resistivity.

Due to decrease in the charge amount of toner with time, as describedabove, the optimum secondary transfer current of an image having, inparticular, a high image area ratio (such as an all-solid single colorimage) in the initial state and the optimum secondary transfer currentafter an elapse of time separate from one another. As a result, theblack toner is overcharged with time, thereby reducing the image densityof the color black. In this case, reducing the primary transfer currentand the secondary transfer current with time is effective as describedabove.

Furthermore, a degree of tolerance of the primary transfer rate and thesecondary transfer rate relative to the transfer current differsdepending on the volume resistivity. If the target primary transfer rateand secondary transfer rate is 90% or more, as one example, when usingtoner having a volume resistivity of 10.9 log Ωcm, the transfer currentin a range of from 9 μA and 24 μA can be used. By contrast, when usingtoner having a volume resistivity of 10.7 log Ωcm, the transfer currentonly in a range of from 9 μA and 21 μA can be used.

The transfer current capable of achieving an optimum transfer ratechanges depending on the charge amount of toner, the electricalpotential of the photosensitive drum, the image area ratio in the mainscanning direction, the resistance of the intermediate transfer belt andthe transfer roller, and so forth. Thus, if the range of usable transfercurrent is narrow, the transfer current needs to be set more precisely.In view of the above, when using the toner having the volume resistivityof equal to or less than 10.7 log Ωcm, it is very effective to adjust orcorrect the transfer current in accordance with changes in the chargeamount of toner with time as in the first illustrative embodiment.

The volume resistivity of toner varies between colors. For example,because color material (pigment) in toner is different for each color,the volume resistivity of toner is different for each color when usingfour different colors of toner such as in the image forming apparatus ofthe first illustrative embodiment. In particular, when the black toneris colored with carbon, the volume resistivity thereof is lower thanthat of other colors of toners such as yellow, magenta, and cyan. Morespecifically, in a case in which carbon black is used as a coloringagent for the black toner, the volume resistivity of the black toner isapproximately 10.7 log Ωcm.

Specific examples of the coloring agent for the toner of the colorsyellow, cyan, and magenta include, but are not limited to, aniline blue,phthalocyanine blue, phthalocyanine green, hansa yellow G, rhodamine 6gLake, calco oil blue, chrome yellow, quinacridone, benzidine yellow,rose bengal, and triarylmethane. These materials can be used alone or incombination. The volume resistivity of these toners is approximately10.9 log Ωcm.

In this case, the charge amount of the black toner is lower than that ofother colors. Thus, in the full-color mode, only the black toner isovercharged easily in the secondary transfer, resulting easily indegradation of an image such as inadequate transfer of black toner andhence thinning the black color in the resulting image. Furthermore, inorder to shorten the first print time in the black single color mode(monochrome mode), in the tandem-type image forming apparatus using anintermediate transfer method, generally, the image forming unit 1K forthe color black is disposed at the extreme downstream end. As describedabove, the toner image in the image forming unit 1K disposed at theextreme downstream end does not get charged up by the primary transfercurrent of other image forming units. The charge amount of toner of thetoner image of the color black in the secondary transfer portion islower than that of other toner images produced in the image formingunits disposed upstream from the image forming unit 1K.

The black toner originally has a low volume resistivity. In addition,because the image forming unit 1K is disposed at the extreme downstreamend, the charge amount of the toner in the secondary transfer portion isrelatively low so that degradation of an image in which the color ofblack in the image appears light occurs easily. Therefore, according tothe illustrative embodiment, adjustment of the transfer current inaccordance with changes in the charge amount of toner with time is veryeffective in a configuration in which the image forming unit 1K of thecolor black is disposed at the extreme downstream end.

Transferring secondarily the toner image having the low charge amountalso causes scattering of toner during conveyance of the recordingmedium. When performing the secondary transfer under the same control asthe conventional transfer current control, toner having the low chargeamount is transferred secondarily onto a recording medium. In general,electrostatic absorption power of toner having a higher charge amountrelative to the recording medium is higher. Thus, the toner having ahigher charge amount is transferred well, hence reducing scattering oftoner. By contrast, the toner having a low charge amount causes easilyscattering of toner on the conveyor guide or the like. Therefore,increasing the primary transfer current is effective in increasing thecharge amount of toner on the intermediate transfer belt. However, aspointed out, the optimum primary transfer current (i.e., the primarytransfer current capable of achieving a high primary transfer rate)shifts towards the lower primary transfer current side when the chargeamount of toner decreases, thereby complicating efforts to increase theprimary transfer current.

In view of the above, reducing the secondary transfer current iseffective in that the charge amount of the recording medium is reduced,thereby getting less susceptible to toner scattering during transfer.Normally, electric discharge in the secondary transfer portion chargesthe recording medium. Thus, with a low secondary transfer current, thecharge amount of the recording medium can be low. The scattering oftoner during conveyance of the recording medium becomes pronounced inthe monochrome mode, which is different from the difference in thesecondary transfer ability for the color black and that of other colors.

The above-described difficulty attributed to the decrease in the chargeamount of toner is pronounced when using the toner with a low volumeresistivity, in particular, the volume resistivity of equal to or lessthan 10.7 log Ωcm. However, by performing the control or adjustmentaccording to the first illustrative embodiment, the toner with a lowvolume resistivity can be treated the same way as the toner with a highvolume resistivity such as conventional toner. Obviously, the toner witha high volume resistivity has similar difficulty more or less. Thus, thecontrol or adjustment according to the first illustrative embodiment iseffective for the toner with a high volume resistivity to prevent imagedefects.

Further, the present disclosure is not limited to these embodiments, butvarious variations and modifications may be made without departing fromthe scope of the present invention. For example, in the firstillustrative embodiment, the primary transfer bias is applied underconstant current control, and the primary transfer bias is adjusted byadjusting a target current value under the constant current control.Alternatively, in a case in which the primary transfer bias is appliedunder constant voltage control the primary transfer bias may be adjustedby adjusting the voltage value. The same can be said of adjustment ofthe secondary transfer bias. The secondary transfer bias may be adjustedby adjusting the current value or the voltage value.

Now, a description is provided of a second illustrative embodiment.

It is to be noted that although an example is provided of control of thetransfer bias using the current value, the same can be said of thecontrol using the voltage value and thus the control is not limitedthereto.

In general, the greater is the transfer current, the higher is thetransfer rate, hence transferring more toner to a transfer medium.Although advantageous, when more of the transfer current is increasedthan necessary, degradation of image quality occurs in primary transferand/or secondary transfer. For example, the transfer rate may drop andthe transferred toner image has uneven image density.

A developing agent deteriorates as image forming operation is repeated,and in general the charge amount (Q/M) of toner in the developing agentdecreases gradually. With the decrease in the charge amount of toner, asdescribed above, the optimum transfer current (i.e., primary transfercurrent) required to transfer the toner image from the photosensitivedrum to the transfer medium changes. Therefore, it is desirable that theprimary transfer current be adjusted or corrected in accordance with thedegree of deterioration of the developing agent to prevent degradationof image quality attributed to deterioration of the developing agent.

FIG. 18 is a graph showing relations between a number of sheets on whichan image is formed (number of printed sheets) and the charge amount oftoner (Q/M). In FIG. 18, changes in the charge amount of toner (Q/M)when continuously forming images having different image area ratios,i.e., 0.5%, 5%, and 20% are shown. As shown in FIG. 18, the lower is theimage area ratio of the image, the more sharply the charge amount oftoner decreases. This is because the lower is the image area ratio, theless is the amount of consumption of toner in the development unit 7.Consequently, a relatively large amount of toner is present in thedevelopment unit 7, thereby increasing stress on the toner.

FIG. 19 is a graph showing relations between a traveling distance of thedeveloping agent and the charge amount of toner (Q/M). Morespecifically, FIG. 19 shows changes in the charge amount of toner (Q/M)when continuously forming images having different image area ratios,i.e., 0.5%, 5%, and 20%. As shown in FIG. 19, not only with the lowimage area ratio (0.5%), but also with the high image area ratio (20%),the charge amount of toner decreases sharply. It is to be noted that thetraveling distance of the developing agent may employ an estimated valueobtained by a process linear velocity (photosensitive drum linearvelocity) multiplied by an operating time of the development unit 7.

As understood from FIGS. 18 and 19, if the primary transfer current isadjusted in accordance with the charge amount of toner based onestimation of changes in the charge amount of toner obtained simply fromthe number of sheets on which an image is formed (number of printedsheets) or the traveling distance of the developing agent, there may bea significant error in the estimated value of the charge amount of tonerdepending on image forming operation conditions (e.g., the difference inthe image area ratio), hindering appropriate adjustment of the primarytransfer current. For this reason, preferably, the degree ofdeterioration of the developing agent takes into consideration of notonly the number of sheets subjected to image formation (number ofprinted sheets) and the traveling distance of the development agent, butalso image forming operation conditions (for example, the difference inthe image area ratio).

According to the second illustrative embodiment, the degree ofdeterioration of the developing agent is obtained using the followingequation, for example.DEGREE OF DETERIORATION OF DEVELOPING AGENT=(TRAVELING DISTANCE OFDEVELOPING AGENT)²/(AMOUNT OF TONER CONSUMPTION)  [EQUATION 3]

FIG. 20 is a graph showing relations between the degree of deteriorationof the developing agent and the charge amount of toner (Q/M) accordingto an illustrative embodiment of the present disclosure.

As understood from FIG. 20, irrespective of the print condition (i.e.,image area ratio), the charge amount of toner (Q/M) decreases at acertain rate. More specifically, FIG. 20 shows changes in the chargeamount of toner (Q/M) when continuously forming images having differentimage area ratios, i.e., 0.5%, 5%, and 20%. As shown in FIG. 20, thecharge amount of toner (Q/M) in any of the image area ratios decreasesin a similar manner. Therefore, using the degree of deterioration of thedeveloping agent obtained in Equation 3, even when the image formingoperation conditions differ, for example, the image area ratio differs,the degree of deterioration of the developing agent which reflectsproperly the change in the charge amount of toner can be obtained. Thus,the primary transfer can be adjusted or corrected in accordance with thedegree of deterioration of the developing agent thus obtained. With thisconfiguration, the primary transfer current is adjusted or correctedproperly irrespective of image forming operation conditions.

The set value for the primary transfer current in the secondillustrative embodiment is calculated using Equation 4.SET VALUE=REFERENCE ELECTRIC CURRENT VALUE×ENVIRONMENT CORRECTIONCOEFFICIENT×ELAPSED TIME CORRECTION COEFFICIENT  [EQUATION 4]

The reference electric current value herein refers to a referenceprimary transfer current value determined by a type, thickness, and soforth of recording medium.

The environment correction coefficient is a correction coefficient dueto changes in ambient conditions including, but not limited totemperature and humidity. According to the present illustrativeembodiment, in order to obtain ambient condition information, atemperature-humidity detector CHS-CSC-18 manufactured by TDK Corporationis used as an environment information obtaining device 90 (shown in FIG.1). The temperature information is obtained from an output of athermistor in the temperature-humidity detector while obtaining humidityinformation from an output of a humidity detector in thetemperature-humidity detector.

Temperature and humidity are detected every minute after the power isturned on. The environment adjustment relative to the reference electriccurrent value is performed in the same timing or on the same detectionperiod as the temperature-humidity detection. It is to be noted thatallocation of the environment information obtaining device 90 is notparticularly limited. Preferably, however, the environment informationobtaining device 90 is disposed spaced apart from a heat source such asthe fixing device 60.

The elapsed time correction coefficient is a correction amount obtainedin accordance with the degree of deterioration of the developing agentobtained by Equation 3.

FIG. 21 is a flowchart showing steps in a process of determination ofthe amount of environment correction amount (environment correctioncoefficient) according to the second illustrative embodiment of thepresent disclosure.

In FIG. 21, at step S31, an output of the thermistor in thetemperature-humidity detector is detected, and a temperature isdetermined using a temperature conversion table in which the thermistoroutput is converted to a temperature based on the correlation betweenthe output of the thermistor and the temperature. Next, at step S32, anoutput of the humidity thermistor in the temperature-humidity detectoris detected, and a relative humidity is determined using the temperatureobtained above and a relative humidity conversion table in which thehumidity detector output is converted to a relative humidity. It is tobe noted that in this table, the temperature is in the row and thehumidity is in the column, and the relative humidity is obtained.

Subsequently, at step S33, based on the relative humidity thus obtainedand an absolute humidity conversion table, an absolute humidity iscalculated. In this table, the relative humidity is in the row and thetemperature is in the column, and the absolute humidity is obtained. Theabsolute humidity can be calculated from the temperature and therelative humidity.

Subsequently, at step S34, based on the absolute humidity thus obtainedand an environment conversion table in which the absolute humidity isconverted to a present environment, the present environment isdetermined. More specifically, in determining the present environment,to which of the following preset environment groups the presentenvironment belongs is determined: for example, L/L (19° C., 30%), M/L(23° C., 30%), M/M (23° C., 50%), M/H (23° C., 80%), H/H (27° C., 80%),and so forth. The combination of the temperature and the humidity in thepreset environment groups are not limited to the above.

Lastly, at step S35, the environment correction coefficient (i.e., theenvironment correction amount) corresponding to the present environmentthus obtained is determined. Detection by the temperature-humiditydetector does not require mechanical operations, thus allowingmonitoring at all times and allowing serial control relative to changesin the ambient conditions.

FIG. 22 is a flowchart showing steps in a process of determination ofthe elapsed time correction amount (elapsed time correction coefficient)according to the second illustrative embodiment of the presentdisclosure.

In the present illustrative embodiment, the elapsed time correctionamount is obtained in accordance with the degree of deterioration of thedeveloping agent obtained by Equation 3. The rate of decrease in thecharge amount of toner indicated by the degree of deterioration of thedeveloping agent depends on various factors that decrease the chargeamount of toner constituting the toner image on the intermediatetransfer belt 41 with time, in addition to deterioration of developingagent and degradation of devices for charging the developing agent.However, the rate of decrease in the charge amount of toner isattributed mainly to mixing of toner in the development unit 7. That is,the main factor is a traveling distance of the developing agent whichcan be estimated by the process linear velocity and an operating time ofthe development unit 7.

Another example of a factor that affects the rate of decrease in thecharge amount of toner is an amount of toner consumption in thedevelopment unit 7. The less is the consumption of toner, the longer thetoner stays in the development unit 7. Consequently, the toner issubjected to repeated contact and friction with the development roller,the photosensitive drum, and so forth, so that deterioration of tonerprogresses. The controller 200 calculates the consumption of toner basedon the image area ratio of the toner images which have been formed.

In the present illustrative embodiment, an index value indicating thedegree of deterioration of the developing agent (rate of decrease in thecharge amount of toner) is obtained using in Equation 3 values of thetraveling distance of the developing agent and the consumption of tonerthat affect the rate of decrease in the charge amount of toner.

According to the present illustrative embodiment, the degree ofdeterioration of the developing agent thus obtained is compared withpredetermined threshold values K1, K2, and K3, and the elapsed timecorrection coefficient (elapsed time correction amount) is determined.The amount of consumption of toner employed in the calculation of thedegree of deterioration of the developing agent corresponds to an amountconsumption of toner used up to the previous image formation. It is tobe noted that the value of the amount of consumption of toner is resetupon replacement of the process cartridge constituting the respectiveimage forming unit.

More specifically, as illustrated in FIG. 22, at step S11, whether ornot the degree of deterioration of the developing agent is less than thethreshold value K1 is determined. If the degree of deterioration is lessthan the threshold value K1 (YES, S11), the elapsed time correctioncoefficient is determined as 100% at step S12. When the degree ofdeterioration is equal to or greater than the threshold value K1 (NO,S11), whether the degree of deterioration of the developing agent isless than the threshold value K2 is determined at step S13. When thedegree of deterioration is determined to be less than the thresholdvalue K2 at step S13 (YES, S13), the elapsed time correction coefficientis determined as 92% at step S14. When the degree of deterioration isequal to or greater than the threshold value K2 (NO, S13), whether thedegree of deterioration of the developing agent is less than thethreshold K2 is determined at step S13. When the degree of deteriorationis determined to be less than the threshold value K3 at step S15 (YES,S15), the elapsed time correction coefficient is determined as 84% atstep S16. When the degree of deterioration is determined to be equal toor greater than the threshold value K3 at step S15 (NO, S15), theelapsed time correction coefficient is determined as 76% at step S16.

According to the second illustrative embodiment, as the thresholdvalues, K1=10000, K2=30000, and K3=70000 are employed. However, thethreshold values are not limited thereto. Using three threshold values,the degree of deterioration of the developing agent is divided into fourgroups, for example. The number of groups is not limited to four.Alternatively, the degree of deterioration may be divided into moregroups or fewer groups. The elapsed time adjustment or correction may beperformed for each print job, or upon reaching a predetermined number ofsheets on which an image is formed, or for every image formation on asheet of recording medium.

Next, a description is now provided of toner usable in the secondillustrative embodiment.

In order to obtain desirable charge on toner through charging in thedevelopment unit and through charging-up in the transfer portion, it hasbeen assumed that using the toner having a volume resistivity greaterthan 10.7 log Ωcm is preferable. However, in recent years, there istoner having the volume resistivity of 10.7 log Ωcm or less for reliablefixability, in particular, at low temperature. Thus, there is demand foran image forming apparatus capable of providing good image quality usingsuch toner.

The toner having a relatively low volume resistivity has acharacteristic in that this type of toner is difficult to keep thecharge. It is assumed that an apparent electrostatic capacity decreaseswhen electrical resistance decreases. Consequently, after the developingagent deteriorated with time, the charge amount of toner having a lowvolume resistivity drops easily as compared with toner having a highvolume resistivity. Due to the decrease in the charge amount of tonerwith time, as described above, the optimum primary transfer current ofan image having, in particular, a high image area ratio (such as anall-solid single color image) in the initial state and the optimumprimary transfer current after an elapse of time separate from oneanother. As a result, the toner is overcharged with time, lowering theimage density. In view of the above, according to the presentillustrative embodiment, the primary transfer current is reduced byusing the above-described elapsed time correction coefficient (elapsedtime correction amount).

The volume resistivity of toner used in the second illustrativeembodiment varies between colors. More specifically, because colormaterial (pigment) in toner is different for each of the colors yellow,cyan, magenta, and black, the volume resistivity of toners differsdepending on the color when using four different colors of toners suchas in the image forming apparatus of the second illustrative embodiment.In particular, because the black toner is colored with carbon, thevolume resistivity of the black toner easily becomes low as comparedwith other toners of the colors (hereinafter referred to as color toner)yellow, magenta, and cyan. More specifically, in a case in which carbonblack is used as a coloring agent for the black toner, the volumeresistivity of the toner is approximately 10.7 log Ωcm.

Specific examples of the coloring agent for the toner of the colorsyellow, cyan, and magenta include, but are not limited to, aniline blue,phthalocyanine blue, phthalocyanine green, hansa yellow G, rhodamine 6gLake, calco oil blue, chrome yellow, quinacridone, benzidine yellow,rose bengal, and triarylmethane. These materials can be used alone or incombination, and the volume resistivity of the toner is approximately10.9 log Ωcm.

In this case, as compared with the color toner, the charge amount of theblack toner drops easily in accordance with the degree of deteriorationof the developing agent. Thus, it is difficult to adjust imaging qualityfor both the black toner and the color toners if the same elapsed timeadjustment is performed on the primary transfer current for the blacktoner and on the primary transfer current for the color toner. In viewof the above, according to the second illustrative embodiment, adifferent elapsed time correction amount is applied to the primarytransfer current for the color black and to the primary transfer currentfor the color toner.

More specifically, for example, the elapsed time correction is performedonly on the primary transfer current for the developing agent for theblack toner, the degree of deterioration of which progresses faster withtime than the developing agent for the color toner. No elapsed timecorrection is performed on the primary transfer current for the colortoner. In this case, the elapsed time correction coefficient isdetermined only for the black toner in accordance with the procedureshown in FIG. 22, while the elapsed time correction coefficient of theprimary transfer current for the color toner is always 100%. This meansthat the elapsed time correction amount of the primary transfer currentfor the color toner is zero.

Alternatively, the elapsed time correction coefficient to be determinedfor the black toner may be less than that of the color toner. That is,the correction amount of the primary transfer current for the colorblack is greater than that of the color toner. For example, the elapsedtime correction coefficient is determined only for the black toner inaccordance with the procedure shown in FIG. 22, and the elapsed timecorrection coefficient for the color toner is determined in accordancewith the procedure shown in FIG. 23.

Alternatively, the threshold values K1, K2, and K3 for changing theelapsed time correction coefficient for the black toner may be less thanthreshold values K1′, K2′, and K3′ for the color toner. In this case,the correction amount of the primary transfer current for the colorblack is also greater than that of the color toner. For example, theelapsed time correction coefficient is determined only for the blacktoner in accordance with the procedure shown in FIG. 22, and the elapsedtime correction coefficient for the color toner is determined inaccordance with the procedure shown in FIG. 24. It is to be noted thatthe relations of the threshold values for the black toner and the colortoner satisfy the following: K1<K1′, K2<K2′, K3<K3′.

As mentioned above, the charge amount of black toner having a low volumeresistivity (in particular, 10.7 log Ωcm or less) drops easily inaccordance with the degree of deterioration of the developing agent ascompared with the color toner. According to the present illustrativeembodiment, the elapsed time correction amount for the black toner isgreater than that for the color toner. In other words, the elapsed timecorrection coefficient for the black toner is less than that for thecolor toner. With this configuration, appropriate adjustment orcorrection is performed on the primary transfer current in accordancewith the rate of decrease in the charge amount of the respective tonershaving different volume resistivity values.

The volume resistivity of the above described toner is obtained byforming toner powder of 3 g into a pellet having a thickness ofapproximately 3 mm by an electric pressing machine and measuring thevolume resistivity of the pellet thus obtained by a TR-10C typedielectric loss measuring instrument (manufactured by Ando Electric Co.,Ltd.), for example.

[Variation 1]

A description is provided of a first variation (Variation 1) of thesecond illustrative embodiment. In the variation 1, as the degree ofdeterioration of the developing agent, a detection result of an imagedensity of a test pattern formed during the image quality adjustmentcontrol (also known as process control) for adjustment of image qualityis used instead of using the degree of deterioration of the developingagent obtained in Equation 3 (i.e., TRAVELING DISTANCE OF THE DEVELOPINGAGENT²/AMOUNT OF TONER CONSUMPTION).

First, a description is provided of the image quality adjustment control(process control) according to the variation 1.

In the image quality adjustment control, a test pattern is formed anddetected so as to adjust an image density and alignment of an image. Theimage quality adjustment control includes an image density control andan alignment adjustment control. In the image density control, forexample, a test pattern for density adjustment (i.e., image qualityadjustment) is formed by developing a predetermined pattern latentimage. A toner adhesion amount, that is, an image density of the testpattern is detected, and based on the detection result, various settingssuch as the toner density in the developing agent in the developmentunit, writing conditions such as exposure power of the optical writingunit 20, a charging bias, and a development bias are adjusted.

In the alignment adjustment control, for example, a test pattern forimage alignment adjustment (i.e., image quality adjustment) is detectedand based on the detection result, writing timing at which a latentimage of each of the toner images is written is adjusted.

The test pattern for the image quality adjustment, i.e., the testpattern for density adjustment, is detected, for example, on thephotosensitive drum between the development area and a primary transferportion, or on the intermediate transfer belt after primary transfer.However, in a case in which the diameter of the photosensitive drum isrelatively small, it is difficult to detect the test pattern on thephotosensitive drum because of installation space for the detector.Therefore, preferably, the test pattern is detected on the intermediatetransfer belt.

The test pattern for the alignment adjustment needs to be detected onthe intermediate transfer belt because it is necessary to detectmisalignment of toner images caused by the difference in the distancebetween the photosensitive drums and different writing timing of thelatent images of each color. In the present illustrative embodiment,both the test pattern for density adjustment and the test pattern forimage alignment adjustment are detected on the intermediate transferbelt.

In general, the image quality adjustment control (process control) isperformed when the power is turned on, before and after print job, andduring a non-image formation period during which no image formingoperation is performed such as after image forming operation on apredetermined number of sheets. For further stability of the imagequality, the test pattern for image quality adjustment may be formed ina non-image area between successive image areas during image formingoperation and detected. An image area refers to an image portion on arecording medium onto which an image is transferred. The image qualityadjustment control during such image forming operation is carried outmostly when controlling a reference value (target toner density) of thetoner density control of the toner density detector.

In the present illustrative embodiment, the test pattern for imagequality adjustment is formed for each color and consists of twopatterns: a horizontal band pattern with a long length in the mainscanning direction and a patch pattern with a short length in the mainscanning direction. An image density (toner adhesion amount) ID of boththe horizontal pattern and the patch pattern is detected for each colorby a detector, and an image density difference ΔID between the imagedensity of the horizontal pattern and the image density of the patchpattern is calculated. The value thus obtained is used as the degree ofdeterioration of the developing agent. As will be described below, thegreater is the image density difference ΔID, the greater is the rate ofdecrease in the charge amount of toner.

More specifically, when performing the primary transfer using an optimumprimary transfer current (initial optimum value) in the initial state inwhich the developing agent has not deteriorated, the primary transferrate of the patch pattern and the primary transfer rate of the solidimage are substantially the same, that is, both approximately 97% in theinitial state. By contrast, after an elapse of time in which thedeveloping agent deteriorated, the primary transfer rate of the patchpattern is approximately 94% while the primary transfer rate of thesolid image is approximately 84%. There is a significant difference inthe primary transfer rate between the patch pattern and the solid image.This indicates that there is a correlation between the charge amount oftoner and the primary transfer rate in that as deterioration of thedeveloping agent progresses, causing the charge amount of toner todecrease, the difference in the primary transfer rate between the patchpattern image and the solid image increases.

Based on this correlation, it is understood that when the difference ΔIDof the image density between the horizontal band pattern and the patchpattern on the intermediate transfer belt obtained from the imagedensity detection result is large, the degree of deterioration of thedeveloping agent (the rate of decrease in the charge amount of toner) islarge.

In the present illustrative embodiment, the horizontal band pattern is a20 mm (vertical)×300 mm, all-solid single color image with a maximumimage density. The patch pattern for each color is a 20 mm (vertical)×10mm, all-solid single color image with a maximum image density. The imagedensity (toner adhesion amount) of these patterns is detected on theintermediate transfer belt 41 by the detector (optical detector). It isto be noted that the test pattern for image quality adjustment tocalculate the degree of deterioration of the developing agent is notlimited to the patterns described above.

FIG. 25 is a flowchart showing example steps in a process ofdetermination of the elapsed time correction amount (elapsed timecorrection coefficient) according to the variation 1.

According to the present illustrative embodiment, the elapsed timecorrection amount is obtained using the difference ΔID of the imagedensity between the horizontal band pattern and the patch pattern asdescribed above. More specifically, as illustrated in FIG. 25, at stepS21, whether or not the difference ΔID is less than a threshold value L1is determined. If the difference ΔID is less than the threshold value L1(YES, S21), the elapsed time correction coefficient is determined as100% at step S22. When the difference ΔID is equal to or greater thanthe threshold value L1 (NO, S21), whether the difference ΔID is lessthan a threshold value L2 is determined at step S23. When the differenceΔID is determined to be less than the threshold value L2 at step S23(YES, S23), the elapsed time correction coefficient is determined as 92%at step S24. When the difference ΔID is equal to or greater than thethreshold value L2 (NO, S23), whether the difference ΔID is less than athreshold value L3 is determined at step S25. When the difference ΔID isdetermined to be less than the threshold value L3 at step S25 (YES,S25), the elapsed time correction coefficient is determined as 84% atstep S26. When the difference ΔID is determined to be equal to orgreater than the threshold value L3 at step S25 (NO, S25), the elapsedtime correction coefficient is determined as 76% at step S27.

According to the present illustrative embodiment, as the thresholdvalues, L1=0.08, L2=0.14, and L3=0.20 are employed. However, thethreshold values are not limited thereto. Using three threshold values,the difference ΔID is divided into four groups, for example. The numberof groups is not limited to four. Alternatively, the difference ΔID maybe divided into more groups or fewer groups.

In the variation 1 of the second illustrative embodiment, as comparedwith the color toner, the charge amount of the black toner drops easilyin accordance with the degree of deterioration of the developing agent.Thus, a different elapsed time correction amount is applied to theprimary transfer current for the color black and to the primary transfercurrent for the color toner. More specifically, similar to the secondillustrative embodiment, for example, the elapsed time correction isperformed only on the primary transfer current for the developing agentfor the black toner, the deterioration of which progresses faster withtime than the developing agent for the color toner. No elapsed timecorrection may be performed on the primary transfer current for thecolor toner.

In another alternative, the elapsed time correction coefficient to bedetermined for the black toner may be less than that of the color toner.In still another alternative, the threshold values L1, L2, and L3 forchanging the elapsed time correction coefficient for the black toner maybe less than threshold values L1′, L2′, and L3′ for the color toner.

[Variation 2]

A description is provided of a second variation of the secondillustrative embodiment (hereinafter referred to as variation 2).

The image forming apparatus (for example, a printer) according to thesecond illustrative embodiment forms an image with at least one colorarbitrarily selected. To simplify a description, a description isprovided of an example of two different modes: a black single color mode(second control mode) and a full-color mode (first control mode). In theblack single color mode, only the toner image formed on thephotosensitive drum 3K of the color black disposed at the extremedownstream end in the moving direction of the intermediate transfer belt41 is transferred primarily onto the intermediate transfer belt 41, andthen transferred secondarily onto a recording medium P, thereby forminga single color image (monochrome image) of the color black. By contrast,in the full-color mode, the toner images formed on all of thephotosensitive drums 3M, 3C, 3Y, and 3K are transferred primarily ontothe intermediate transfer belt 41 such that they are superimposed oneatop the other, forming a four-color composite toner image. Thefour-color composite toner image is secondarily transferred onto arecording medium P, thereby forming a full color image on the recordingmedium.

In the variation 2, the primary transfer bias in the image forming unit1K for the color black disposed at the extreme downstream end in themoving direction of the intermediate transfer belt 41 is reduced insteps in the black single color mode. In the full-color mode, bycontrast, the primary transfer bias is increased in steps.

As described above, in the event in which the charge amount of overalltoner has dropped due to deterioration of the developing agent withtime, the set value of the primary transfer current is reduced toprevent degradation of the primary transfer rate. In either the blacksingle color mode or the full-color mode, preferably, the primarytransfer bias for the color black is reduced in stages in terms of theprimary transfer rate. However, the image density of the color black inan output image depends on the primary transfer rate so that it isnecessary to take both the primary transfer rate and the secondarytransfer rate into consideration with balance.

Thus, in the full-color mode, multiple toner images are transferred ontothe intermediate transfer belt 41 such that they are superimposed oneatop the other, forming a composite toner image. The composite tonerimage thus obtained needs to be transferred secondarily from theintermediate transfer belt 41 onto a recording medium P. By contrast, inthe black single color mode in which only one of the image bearingmembers (hereinafter referred to as downstream image bearing member)used in the full-color mode is used, one toner image (black color)without other toner images superimposed thereon is transferredsecondarily from the intermediate transfer belt 41 to the recordingmedium P. As a result, an amount of toner to be transferred secondarilyto the recording medium P at the secondary transfer portion is greaterin the full-color mode than in the black single color mode. Therefore,an optimum secondary transfer bias to achieve an optimum secondarytransfer rate is greater in the full-color mode than in the black singlecolor mode. Thus, the secondary transfer bias is set greater in thefull-color mode than in the black single color mode.

At this time, if the primary transfer current is reduced in accordancewith the degree of deterioration of the developing agent, the primarytransfer rate is enhanced. However, since the charge amount of overalltoner is relatively low due to deterioration of the developing agent andhence the primary transfer current gets low, the charge amount of tonerat the secondary transfer portion is even lower than before adjustmentor correction. Degradation of image quality attributed to the decreasein the charge amount of toner at the secondary transfer portion isgreater in the full-color mode in which the secondary transfer bias isrelatively high than in the black single color mode in which thesecondary transfer bias is relatively low.

The relations between the secondary transfer current and the secondarytransfer rate may be considered as having substantially the samerelations as between the primary transfer current and the primarytransfer rate. That is, during which the amount of toner moving from thephotosensitive drum side to the intermediate transfer belt side at thesecondary transfer portion increases with an increase in the secondarytransfer bias, flow of electric current caused by the movement of tonerincreases, hence increasing the secondary transfer current. By contrast,after the amount of movement of toner reaches a state of saturation, theflow of electric current caused by the movement of toner stopsincreasing. Consequently, the electrical discharge at the secondarytransfer portion increases in accordance with an increase in thesecondary transfer bias. In this case, the secondary transfer currentincreases with an increase in the electrical discharge. However, thesecondary transfer rate decreases with the increase in the electricaldischarge.

The value of the secondary transfer current in the full-color mode isset to achieve a highest possible secondary transfer rate within a rangein which the secondary transfer rates for each of the plurality of tonerimages constituting the composite toner image are approximately the same(that is, none of the secondary transfer rates has a significant lowvalue relative to all the other ratios). The toner image to betransferred primarily from the photosensitive drum disposed in theupstream side in the surface moving direction of the intermediatetransfer belt among the plurality of toner images constituting thecomposite toner image is charged up with the primary transfer currentwhen passing through the primary transfer portion in the downstreamtherefrom. As a result, the charge amount of toner in the secondarytransfer portion is higher than that of the toner image to betransferred primarily from the downstream photosensitive drum.

In a case in which the secondary transfer current for transferringsecondarily the plurality of toner images having different chargeamounts all at once is determined as described above, for the tonerimage with a relatively low charge amount (the toner image transferredprimarily from the downstream photosensitive drum), the secondarytransfer current is set to a higher value than a value (peak value)achieving the maximum secondary transfer rate.

That is, in the full-color mode, the secondary transfer current for theblack toner image in the downstream is set to a higher value than avalue (peak value) achieving the maximum secondary transfer rate.

By contrast, in the black single color mode for forming only the blacktoner image, because there is one toner image to be formed, that is, theblack toner image, the secondary transfer current is set to achieve theoptimum secondary transfer rate for the black toner image.

In a case in which the primary transfer current for the image formingunit 1K disposed at the extreme downstream end is reduced in accordancewith the degree of deterioration of the developing unit in thefull-color mode and the black single color mode, with the decrease inthe primary transfer current for the black toner image the charge amountof toner at the secondary transfer portion becomes lower than thatbefore the adjustment. At this time, as the charge amount of toner inthe secondary transfer portion decreases, the flow of electric currentcaused by the movement of the toner at the secondary transfer portiondecreases. Thus, when the amount of movement of toner reaches the stateof saturation the secondary transfer current (i.e., when the secondarytransfer rate reaches its peak) is less than that before the adjustment.As a result, the relations between the secondary transfer current andthe secondary transfer rate after adjustment or correction of theprimary transfer current shift towards the lower secondary transfercurrent side.

The rate of change in the secondary transfer rate relative to the changein secondary transfer current tends to increase when the secondarycurrent separates from the peak value for achieving the maxim secondarytransfer rate. That is, in the full-color mode, the set value of thesecondary transfer current in the full-color mode before adjustment isshifted towards the higher secondary transfer current than the peakvalue for achieving the maximum secondary transfer rate. As a result,when the peak value shifts toward the lower secondary transfer currentside due to adjustment of the primary transfer current, the set value ofthe secondary transfer current separates further away from the peakvalue for achieving the maximum secondary transfer rate. As a result, inthe full-color mode, the adjustment of the primary transfer current forthe color black results in a significant decrease in the secondarytransfer rate.

By contrast, as described above, in the black single color mode, the setvalue of the secondary transfer current for the black toner image beforethe adjustment is set to be near the peak value for achieving themaximum secondary transfer rate for the color black. With thisconfiguration, even when the peak value shifts toward the lowersecondary transfer current side after the adjustment of the primarytransfer current, the set value of the secondary transfer current doesnot separate from the peak value for achieving the maximum secondarytransfer rate. Thus, the decrease in the secondary transfer rate for thecolor black due to the adjustment of the primary transfer current forthe color black is less than the full-color mode.

In the second illustrative embodiment, in the black single color mode,the primary transfer current is reduced in accordance with the degree ofdeterioration of the developing agent. By contrast, in the full-colormode, the primary transfer current is increased in accordance with thedegree of deterioration of the developing agent. With thisconfiguration, in the black single color mode, there is less influenceof the decrease in the secondary transfer rate attributed to theadjustment of the primary transfer current on image quality, and hencethe decrease in the primary transfer rate attributed to thedeterioration of the developing agent is corrected by adjusting theprimary transfer current. Accordingly, degradation of image quality issuppressed, if not prevented entirely.

In the full-color mode, with regards to the degradation of imagequality, there is more influence of the decrease in the secondarytransfer rate attributed to the adjustment of the primary transfercurrent in which the primary transfer current is reduced than of thedecrease in the primary transfer current attributed to the deteriorationof the developing agent. According to the present illustrativeembodiment, in the full-color mode, the primary transfer current for thecolor black is increased in accordance with deterioration of thedeveloping agent. Although the primary transfer rate for the color blackmay not be enhanced, the charge amount of the black toner at thesecondary transfer portion is increased, thereby enhancing the secondarytransfer rate for the color black. As a result, the image density of thecolor black is prevented from decreasing in the full-color mode.

[Variation 3]

A description is provided of a third variation of the secondillustrative embodiment (hereinafter referred to as variation 3).

According to the present illustrative embodiment, in the full-colormode, the frequency of use of the black toner is reduced at a blackimage portion as the degree of deterioration of the developing agentincreases, and use of composite black or process black made with tonersof cyan, magenta, and yellow is increased instead.

As described above, in the full-color mode, when the charge amount ofblack toner decreases due to the deterioration of the developing agent,not only the primary transfer rate, but also the secondary transfer ratedrops significantly. If the primary transfer current for black color isreduced to improve the primary transfer rate, the secondary transferrate for black color drops significantly, thereby reducing easily theimage density of the black color. Even when the charge amount of tonerdecreases due to deterioration of the developing agent, the toner imagesof yellow, cyan, and magenta to be transferred from the image formingunits 1Y, 1C, and 1M disposed upstream from the image forming unit 1K ofthe black color in the surface moving direction of the intermediatetransfer belt 41 are charged up with the primary transfer current forthe color black when passing at least through the image forming unit 1Kand hence an adequate amount of charge is maintained on toner at thesecondary transfer portion.

As described above, in the present illustrative embodiment, the tonersof yellow, cyan, and magenta have a higher volume resistivity than thatof the black toner and are easily charged. Thus, the charge amount oftoner thereof at the secondary transfer portion is higher than that ofthe black toner.

According to the present illustrative embodiment, taking advantage ofthese characteristics, an image of the color black is formed morefrequently with the process black made with toners of cyan, magenta, andyellow having a small rate of decrease in the transfer rate even whenthe degree of deterioration of the developing agent is significant. Withthis configuration, the image density of the black color is preventedfrom decreasing.

[Variation 4]

A description is provided of a fourth variation of the fourthillustrative embodiment (hereinafter referred to as variation 4).

In a case in which the charge amount of black toner has decreased due tothe deterioration of the developing agent, not only the primary transferrate, but also the secondary transfer rate drops easily. In this state,reducing the secondary transfer current can enhance the secondarytransfer rate. More specifically, in a case in which the primarytransfer current is reduced, hence reducing further the charge amount oftoner at the secondary transfer portion, the secondary transfer currentis reduced in return to prevent degradation of image quality as comparedwith performing no adjustment on the secondary transfer current.Reducing the secondary transfer current can also prevent degradation ofimage quality attributed to a residual image and enhance the life ofparts used for the transfer process.

[Variation 5]

A description is provided of a fifth variation of the secondillustrative embodiment (hereinafter referred to as variation 5).

The set value for the primary transfer current in the presentillustrative embodiment is calculated using Equation 5. In the presentillustrative embodiment, the environment correction coefficient ischanged in accordance with the degree of deterioration of the developingagent.SET VALUE=REFERENCE ELECTRIC CURRENT VALUE×ENVIRONMENT CORRECTIONCOEFFICIENT  [EQUATION 5]

An example control of the present illustrative embodiment is shown inFIG. 26. In FIG. 26, the deterioration of the developing agent iscategorized into three groups in the similar or the same manner aschanging the elapsed time correction coefficient of the primary transfercurrent.

[Variation 6]

A description is provided of a sixth variation of the secondillustrative embodiment (hereinafter referred to as variation 6).

In the present illustrative embodiment, electrical resistance of a paththrough which the primary current flows upon primary transfer isdetected by electrical resistance detectors 80Y, 80M, 80C, and 80K(shown in FIG. 1), and the elapsed time correction coefficient isdetermined in consideration of the electrical resistance thus obtained.The electrical resistance of transfer devices such as the primarytransfer rollers 45Y, 45C, 45Y, and 45K, and the intermediate transferbelt 41 contributes largely to the primary transfer rate. Morespecifically, if the electrical resistance of these devices is too low,an influence of the electrical resistance of a toner layer in theprimary transfer portion becomes pronounced and the primary transferbias changes significantly depending on the image area ratio. In otherwords, the primary transfer rate varies depending on the image arearatio.

By contrast, in a case in which the electrical resistance of thetransfer devices is too high, leakage of electrical current becomes toohigh, resulting in image defects and an increase in voltage near theupper limit of the power source, hence hindering flow of the electriccurrent. When this occurs, transfer is not performed properly and thepower source may be damaged.

The electrical resistance of the transfer devices such as theintermediate transfer belt 41 and the primary transfer rollers 45Y, 45C,45Y, and 45K (the electrical resistance material on the primary transfercurrent path) changes often with time. In view of the above, in thepresent illustrative embodiment, with the change in the electricalresistance of these transfer devices, the elapsed time correctioncoefficient is changed, thereby adjusting properly the primary transfercurrent to accommodate changes in the electrical resistance of thetransfer devices.

In the second illustrative embodiment, as described above, the powersource which supplies the primary transfer voltage to the primarytransfer rollers 45Y, 45C, 45Y, and 45K is under constant currentcontrol. Thus, by detecting the primary transfer voltage supplied to theprimary transfer rollers 45Y, 45C, 45Y, and 45K, the electricalresistance of the transfer devices can be detected. It is to be notedthat the voltage supplied to the primary transfer rollers 45Y, 45C, 45Y,and 45K is detected. However, detection of the voltage is not limited tothe primary transfer rollers. For example, only the voltage supplied tothe intermediate transfer belt 41 may be detected. Alternatively, thevoltage of at least one of the primary transfer rollers 45Y, 45C, 45Y,and 45K, and the intermediate transfer belt 41 may be detected.

In the present illustrative embodiment, in a case in which an electriccurrent used to detect the electrical resistance is, for example,approximately 25 μA, the applied voltage of the primary transfer rollers45Y, 45C, 45Y, and 45K and the electrical resistance thereof haverelations as shown in FIG. 27. As understood from FIG. 27, the primarytransfer voltage differs depending on the electrical resistance of theprimary transfer roller. That is, the higher is the electricalresistance, the higher is the primary transfer voltage. Therefore, thedetection of the primary transfer voltage allows understanding of theresistance of the primary transfer roller.

FIG. 28 is a table showing an example of relations between theelectrical resistance of the primary transfer roller and a primarytransfer current (an optimum current) capable of achieving a maximumprimary transfer rate.

As can be understood from FIG. 13, where the electrical resistance ofthe primary transfer roller as a reference is approximately ten to thepower of 7.5 (1×10^(7.5)Ω) the appropriate primary transfer current is25 μA. When the electrical resistance of the primary transfer roller isapproximately ten to the power of 7.0, the appropriate primary transfercurrent is 29 μA. Therefore, when the electrical resistance of theprimary transfer roller is changed from ten to the power of 7.5 to thepower of 7.0, the set value of the primary transfer current is adjustedby an adjustment amount of +4 μA. Therefore, preferably, when theelectrical resistance of the primary transfer roller is changed from tento the power of 7.5 to the power of 8.0, the set value of the primarytransfer current is adjusted by an adjustment amount of −4 μA.

When the electrical resistance of the primary transfer was ten to thepower of 9.0, the set value 21 μA of the primary transfer current causedan image failure attributed to electric discharge. At this time, whenthe set value of the primary transfer current was 17 μA, no imagefailure occurred. When the set value of the primary transfer current iseither 21 μA or 17 μA, there is not much difference in the transferrate. Thus, when the electrical resistance of the primary transferroller is ten to the power of 9.0, preferably, the set value of thetransfer current is adjusted by an adjustment amount of −8 μA.

In a case in which the primary transfer current is adjusted inaccordance with the electrical resistance of the primary transfer rolleras described above, the amount of adjustment or correction of theelapsed time correction coefficient is selected based on whether or notthe primary transfer voltage detected is lower or higher than thepredetermined threshold voltage value. FIG. 29 is a table showing anexample of relations between the detected primary transfer voltage andthe time correction coefficient after being changed in accordance withthe detected primary transfer voltage.

In the present illustrative embodiment, the primary transfer current canbe adjusted or corrected in accordance with a change in the electricalresistance of the transfer devices, thereby achieving more optimumprimary transfer rate than when adjusting the primary transfer currentin accordance with the change in the charge amount of toner attributedto the deterioration of the developing agent.

Although advantageous, the detection of voltage as described abovenecessitates mechanical operations such as reading a voltage bysupplying a transfer current for a certain period of time, hencereducing the productivity of the machines due to operations associatedwith the detection of voltage. However, the image quality adjustmentcontrol (i.e., process control) is often performed during the non-imageformation period so that performing the above-described detection ofvoltage during the image quality adjustment control prevents degradationof the productivity of the machines due to performing the detection ofvoltage alone. Therefore, in the present illustrative embodiment, thevoltage is detected during the image quality adjustment control.

It is to be noted that in the present illustrative embodiment, anexample has been given of changes in the primary transfer roller.Alternatively, since the electrical resistance of the intermediatetransfer belt 41 changes in a similar manner as that of the primarytransfer roller, the primary transfer roller current may be adjusted orcorrected in accordance with the electrical resistance of theintermediate transfer belt 41.

In a case in which the primary transfer bias is applied under constantvoltage control, not under constant current control, the electricalresistance of the transfer devices is detected by detecting the primarytransfer current.

The above descriptions have been provided of the second illustrativeembodiment and the variations. Various modifications and alterations ofthis disclosure will become apparent to those skilled in the art withoutdeparting from the scope and principles of this disclosure, and itshould be understood that this disclosure is not to be unduly limited tothe illustrative embodiments set forth herein. For example, thedetection of the degree of deterioration of the developing agent and thedetermination of whether the degree of deterioration of the developingagent has reached a level requiring the elapsed time correctionadjustment of the primary transfer current are not performed on allimage forming units in the image forming apparatus in view ofsimplification of control, but are performed only on the image formingunit in use. The primary transfer current is controlled using a voltagevalue, instead of a current value. The developing agent is either asingle-component developing agent consisting of toner or a two-componentdeveloping agent consisting of toner and carrier. The environmentdetector may be provided to each image forming unit.

According to the second illustrative embodiment, the image formingapparatus employs a so-called intermediate transfer method in which thetoner image formed on the photosensitive member is transferred primarilyonto the intermediate transfer belt, and then transferred onto arecording medium. Alternatively, the image forming apparatus may employa direct transfer method in which the toner image formed on thephotosensitive member is transferred directly onto a recording medium asillustrated in FIG. 30. FIG. 30 is a cross-sectional diagramschematically illustrating an image forming apparatus of the directtransfer method.

The image forming apparatus to which the variations 2 through 4 can beapplied is not limited to the image forming apparatus described above.As illustrated in FIG. 30, transfer portions are formed between atransfer belt 335 and each of the four photosensitive drums 3M, 3C, 3Y,and 3K. The transfer belt 335 is formed into a loop. A bias roller 335 aand an auxiliary roller 335 b are disposed near or contact an innercircumferential surface of the transfer belt 335 at the transferportions. Each of the bias rollers 335 a is connected to a transfer biaspower source 339 which applies a transfer bias to each of the biasrollers 335 a. FIG. 30 illustrates only the transfer bias power source339 corresponding to the photosensitive drum 3M for the color magenta,and the transfer bias power sources for other photosensitive drums areomitted herein for simplicity.

Furthermore, the image forming apparatus to which the first and thesecond illustrative embodiments are applied includes, but is not limitedto a so-called tandem-type image forming apparatus and a single-drumtype image forming apparatus such as shown in FIG. 17 in which tonerimages are formed on a single photosensitive member and are transferredonto a transfer medium such that they are superimposed one atop theother, forming a color image.

As illustrated in FIG. 17, the image forming apparatus includes abelt-type photosensitive member 203 serving as an image bearing member.Four development devices 206M, 206C, 206Y, and 206B are disposed aroundthe photosensitive member 203 to develop latent images using respectivecolors of toner. A charging device 204, a writing unit 205, a primarytransfer roller 207, and a cleaning device 208 are common to all colors.In this configuration, when forming an image in the full-color mode,first, a magenta toner image is formed on the photosensitive member 203and is transferred onto an intermediate transfer belt 202 at a primarytransfer portion. Subsequently, when the primarily transferred magentatoner image arrives again at the primary transfer portion, a cyan tonerimage formed on the photosensitive member 203 is primarily transferredover the magenta toner image on the intermediate transfer belt 202. Thisprocess is repeated for the toner images of the colors yellow and black,thereby forming a composite toner image on the intermediate transferbelt 202 similar to the tandem-type transfer method. The composite tonerimage is transferred onto a recording medium by a secondary transferdevice 209 at the secondary transfer portion, thereby forming a colorimage on the recording medium.

The above-described image forming apparatus is an example of the imageforming apparatus of the present disclosure. The present disclosureincludes the following aspects.

[Aspect A]

According to an aspect A, an image forming apparatus includes anintermediate transfer member (i.e., intermediate transfer member 41) tomove in a first direction; a plurality of image bearing members (i.e.,photosensitive drums 3Y, 3C, 3M, 3K) to bear toner images thereon, theplurality of image bearing members disposed along the first direction; aplurality of toner image forming devices (i.e., development devices 7Y,7C, 7M, and 7K) to form the toner images on the plurality of imagebearing members using different developing agents; a primary transferdevice (i.e., primary transfer rollers 45Y, 45C, 45M, and 45K) to applya primary transfer bias to primarily transfer each of the toner imagesformed on the plurality of image bearing members onto a surface of theintermediate transfer member to form a composite toner image; asecondary transfer device (i.e., secondary transfer auxiliary roller 46)to apply a secondary transfer bias to secondarily transfer the compositetoner image formed on the intermediate transfer member to a recordingmedium; a controller (i.e., controller 200) to selectively control imageforming operation between a first mode and a second mode such that inthe first mode the composite toner image is formed using at least two ofthe plurality of image bearing members including a first image bearingmember (i.e., photosensitive drum 3K) and a second image bearing memberand after the composite toner image is formed on the intermediatetransfer member the secondary transfer bias is applied to secondarilytransfer the composite toner image from the intermediate transfer memberto the recording medium, and in the second mode the toner image isformed on the first image bearing member used in the first mode which isdisposed downstream from the second image bearing member in the firstdirection and after the toner image is primarily transferred from thefirst image bearing member onto the intermediate transfer member thesecondary transfer bias less than that in the first mode is applied totransfer the toner image from the intermediate transfer member to therecording medium; a first developing agent condition detector (i.e.,developing agent condition detector 200K) to detect a degree ofdeterioration of a developing agent used to form the toner image on thefirst image bearing member; and a primary transfer bias adjuster (i.e.,primary transfer bias adjuster 200 a) to adjust, in the second mode, theprimary transfer bias by a correction amount in accordance with thedegree of deterioration of the developing agent detected by the firstdeveloping agent condition detector so as to reduce a primary transfercurrent of the primary transfer bias upon transferring the toner imagefrom the first image bearing member onto the intermediate transfermember, and to adjust, in the first mode, the primary transfer bias by acorrection amount less than the correction amount in the second mode ornot to adjust the primary transfer bias in accordance with the degree ofdeterioration of the developing agent detected by the first developingagent condition detector upon primarily transferring the toner imagefrom the first image bearing member onto the intermediate transfermember.

With this configuration, degradation of image quality is prevented inthe first mode (full-color mode) using the plurality of image bearingmembers, while preventing degradation of the primary transfer rateattributed to the deterioration of the developing agent in the secondmode (black single color mode) using one image bearing member.

[Aspect B]

According to the aspect A, the first image bearing member is disposed atan extreme downstream end in the first direction. The toner imageprimarily transferred from the image bearing member disposed at theextreme downstream end onto the intermediate transfer member is conveyedto the secondary transfer portion without getting charged up by otherimage bearing members, and the charge amount of the toner is at thelowest at the secondary transfer. Thus, the present disclosure iseffective.

[Aspect C]

The image forming apparatus of the aspect B further includes a seconddeveloping agent condition detector (i.e., developing agent conditiondetectors 200Y, 200C, and 200M) to detect deterioration of thedeveloping agent in the toner image formed on the image bearing memberused in the first mode other than the first image bearing member. Theprimary transfer bias adjuster adjusts at least in the first mode theprimary transfer bias for the image bearing member used in the firstmode other than the first image bearing member by the correction amountcorresponding to the degree of deterioration of the developing agentdetected by the second developing agent condition detector, and thecorrection amount is greater than that for the first image bearingmember.

Because the toner image primarily transferred from the image bearingmember other than the first image bearing member is charged up at leastby the first image bearing member before being conveyed to the secondarytransfer portion, the decrease in the charge amount of toner of thetoner image is compensated by the charge-up with the primary transfercurrent by the first image bearing member even when the primary transfercurrent is reduced to improve the primary transfer rate. Thus, thesecondary transfer rate is not degraded at the secondary transferportion. By contrast, the toner image primarily transferred from theimage bearing member disposed at the extreme downstream end onto theintermediate transfer member is conveyed to the secondary transferportion without getting charged up by other image bearing member, and inorder to reduce the primary transfer current to enhance the primarytransfer rate the charge amount of toner drops, hence affectingsignificantly the secondary transfer rate. Therefore, in terms of thesecondary transfer rate, the amount of adjustment of the primarytransfer current for the first image bearing member should not be large.According to the present aspect, the amount of adjustment of the primarytransfer current for the first image bearing member is greater than thatfor other image bearing members, thereby providing well balanced imagequality.

[Aspect D]

According to any one of aspects A through C, the image forming apparatusincludes a secondary transfer bias adjuster to adjust the secondarytransfer bias by an amount in accordance with the correction amount ofthe primary transfer bias by the primary transfer bias adjuster. Thisconfiguration provides good imaging quality over time from the initialstate.

[Aspect E]

According to the aspect D, the amount of correction of the secondarytransfer bias employed by the secondary transfer adjuster is greater inthe second mode than in the first mode. This configuration provides goodimaging quality in any of the modes.

[Aspect F]

According to any one of aspects A through D, the image forming apparatusincludes a secondary transfer bias adjuster to adjust the secondarytransfer bias by an amount in accordance with the degree ofdeterioration of the developing agent detected by the developing agentcondition detector. This configuration provides good imaging qualityover time from the initial state.

[Aspect G]

According to any one of aspects A through F, the image forming apparatusincludes a secondary transfer bias adjuster to adjust the secondarytransfer bias in accordance with an area of the toner image transferredfrom the second image bearing member disposed upstream from the firstimage bearing member in the first direction onto the intermediatetransfer member. This configuration provides good imaging qualityregardless of the image area.

[Aspect H]

According to the aspect G, the secondary transfer bias adjuster adjuststhe secondary transfer bias in accordance with the area of the tonerimage and the degree of deterioration of the developing agent. Thisconfiguration provides good imaging quality over time from the initialstate.

[Aspect I]

According to the aspect H, the secondary transfer bias adjuster adjuststhe secondary transfer bias such that in a case in which the area of thetoner image is less than a predetermined area, a secondary transfercurrent is reduced with an increase in the degree of deterioration ofthe developing agent.

[Aspect J]

According to any one of aspects A through I, the first image bearingmember bears a toner image formed with a black toner. This configurationenhances the image quality using the black toner.

[Aspect K]

According to an aspect K, an image forming apparatus includes an imagebearing member (i.e., photosensitive drum 203) to rotate; anintermediate transfer member (i.e., intermediate transfer belt 202) tomove in a first direction; a plurality of toner image forming devices(i.e., development devices 206M, 206C, 206Y, and 206B) to formsequentially and overlappingly a plurality of toner images usingdifferent developing agents on a surface of the image bearing member toform a composite toner image; a primary transfer device (i.e., primarytransfer roller 207) to apply a primary transfer bias to primarilytransfer the composite toner image formed on the image bearing memberonto a surface of the intermediate transfer member; a secondary transferdevice (i.e. secondary transfer device 209) to apply a secondarytransfer bias to secondarily transfer the composite toner image havingbeen primarily transferred on the intermediate transfer member onto arecording medium; a controller (i.e., controller 200) to selectivelycontrol image forming operation between a first mode (i.e., full-colormode) and a second mode (i.e., black single color mode) such that in thefirst mode the composite toner image is formed using at least two of theplurality of toner image forming devices including a first toner imageforming device and a second toner image forming device and after thecomposite toner image is primarily transferred onto the intermediatetransfer member the secondary transfer bias is applied to secondarilytransfer the composite toner image from the intermediate transfer memberonto the recording medium, and in the second mode the first toner imageforming device (i.e., development device 206B) used in the first modeforms the toner image (black toner image) which is transferred after thetoner image formed by the toner image forming device other than thefirst toner image forming device is transferred and after the tonerimage formed by the first toner image forming device is primarilytransferred onto the intermediate transfer member the secondary transferbias less than that in the first mode is applied to transfer the tonerimage from the intermediate transfer member onto the recording medium; adeveloping agent condition detector (i.e., developing agent conditiondetectors 200K, 200C, 200M, and 200Y) to detect a degree ofdeterioration of a developing agent used to form the toner image on thefirst toner image forming device; and a primary transfer bias adjuster(i.e., primary transfer bias adjuster 200 a) to adjust the primarytransfer bias by a correction amount in accordance with the degree ofdeterioration of the developing agent detected by the developing agentcondition detector so as to reduce a primary transfer current of theprimary transfer bias upon transferring the toner image formed by thefirst toner image forming device in the second mode, and to adjust theprimary transfer bias by the correction amount less than the correctionamount in the second mode or not to adjust the primary transfer bias inaccordance with the degree of deterioration of the developing agentdetected by the developing agent condition detector upon primarilytransferring the toner image formed by the first toner image formingmember in the first mode.

With this configuration, in the tandem-type image forming apparatus,degradation of image quality is prevented in the first mode (full-colormode) while improving the decrease in the primary transfer rateattributed to the deterioration of developing agent in the second mode(black single color mode) using one toner image.

[Aspect L]

According to an aspect L, an image forming apparatus includes aplurality of image bearing members (i.e., photosensitive drums 3Y, 3C,3M, 3K) to rotate in a first direction; a plurality of toner imageforming devices (i.e., development devices 7M, 7C, 7Y, and 7K) to form atoner image on a surface of each of the plurality of image bearingmembers with developing agents including toners having different volumeresistivities; a plurality of transfer devices (i.e., primary transferrollers 45M, 45C, 45Y, and 45K) to apply a transfer bias to transfer thetoner images formed on the plurality of image bearing members onto atransfer medium to form a composite toner image; a developing agentcondition detector (i.e., developing agent condition detectors 200K,200C, 200M, and 200Y) to detect a degree of deterioration of thedeveloping agents; and a transfer current adjuster (i.e., primarytransfer bias adjuster 200 a) to adjust a transfer current of thetransfer bias by a correction amount in accordance with the degree ofdeterioration of the developing agent detected by the developing agentcondition detector upon transferring the toner images formed on at leasttwo image bearing members, the toner images being formed with thedeveloping agents including the toners having different volumeresistivities. The correction amount is different between the at leasttwo image bearing members. That is, the correction amount for thephotosensitive drum 3B using the black toner is different from that forthe photosensitive drums 3Y, 3C, and 3M.

With this configuration, appropriate adjustment of the transfer currentis performed in accordance with the volume resistivity of toners whenforming toner images using different developing agents having toners ofdifferent volume resistivities on at least image bearing members.

[Aspect M]]

According to the aspect L, the development agent condition detectordetects the degree of deterioration of the developing agent based on anamount of toner (amount of toner consumption) used by the plurality ofthe toner image forming devices in a predetermined time period.

The less is the amount of toner consumed by the toner image formingdevice within a predetermined time period, the faster that the tonerdeteriorates. The degree of deterioration of the developing agent can bedetected reliably with this configuration.

[Aspect N]

According to aspects L or M, a plurality of toner patterns having a samelength in the first direction and different lengths in a width directionperpendicular to the first direction of the image bearing members areformed in a non-image forming area of the at least two image bearingmembers for adjustment of image quality, and the development agentcondition detector detects the degree of deterioration of the developingagent by obtaining a difference ΔID in image densities of the pluralityof toner patterns based on a detection result of the image densities ofthe plurality of toner patterns.

With this configuration, the degree of deterioration of the developingagent can be detected using the patterns so that a designated device fordetection of the degree of deterioration of the developing agent is notnecessary.

[Aspect O]

According to any one of aspects L through N, the image forming apparatusincludes an intermediate transfer member (i.e., intermediate transferbelt 2) to move in a second direction as the transfer medium; asecondary transfer device (i.e., secondary transfer device 9) to apply asecondary transfer bias to secondarily transfer the composite tonerimage from the intermediate transfer member onto a recording medium; anda controller (i.e., controller 200) to selectively control image formingoperation between a first mode (full-color mode) and a second mode(i.e., black single color mode) such that in the first mode thecomposite toner image is formed using the at least two image bearingmembers including a first image bearing member and a second imagebearing member and after the composite toner image is primarilytransferred onto the intermediate transfer member the secondary transferbias is applied to secondarily transfer the composite toner image fromthe intermediate transfer member to the recording medium, and in thesecond mode the toner image is formed on the first image bearing member(i.e., photosensitive drum 3K) used in the first mode which is disposeddownstream from the second image bearing member in the second directionof the intermediate transfer member and after the toner image isprimarily transferred from the first image bearing member onto theintermediate transfer member the secondary transfer bias less than thatin the first mode is applied to transfer the toner image from theintermediate transfer member onto the recording medium. In the secondmode the transfer current adjuster reduces a primary transfer current ofa primary transfer bias with an increase in the degree of deteriorationof the developing agent upon primarily transferring the toner image fromthe first image bearing member onto the intermediate transfer member,while increasing in the first mode the primary transfer current of theprimary transfer bias with an increase in the degree of deterioration ofthe developing agent upon primarily transferring the toner image fromthe first image bearing member onto the intermediate transfer member.

With this configuration, in the second mode in which the secondarytransfer rate is less affected by the adjustment (reduction) of theprimary transfer current, the primary transfer current is reduced withan increase in the degree of deterioration of the developing agent toenhance the primary transfer rate, thereby suppressing degradation ofimage quality. In the first mode in which the secondary transfer rate issignificantly affected by the adjustment (reduction) of the primarytransfer current, the primary transfer current is increased with anincrease in the degree of deterioration of the developing agent toenhance the secondary transfer rate, thereby suppressing degradation ofimage quality.

[Aspect P]

According to any one of aspects L through N, the image forming apparatusincludes an intermediate transfer member to move in a second directionas the transfer medium; a secondary transfer device to apply a secondarytransfer bias to secondarily transfer the composite toner image from theintermediate transfer member onto a recording medium; and a controllerto control image forming operation in a control mode such as afull-color mode in which the toner images are formed on at least threeimage bearing members including at least the first image bearing memberand the second image bearing member and are primarily transferred ontothe intermediate transfer member such that they are superimposed oneatop the other to form the composite toner image which is transferredonto the recording medium. In the control mode, the controller controlsthe image forming operation such that with an increase in the degree ofdeterioration of the developing agent having a first toner (black toner)used in the toner image formed on one of the at least three imagebearing members disposed at a downstream side in the second direction ofthe intermediate transfer member to express a first color such as thecolor black in an image, a ratio of use of other toners (the colors ofyellow, magenta, and cyan) used in the toner images formed on at leasttwo other image bearing members upstream from the image bearing memberused for the first color is increased to express the first color withoutusing the first toner.

With this configuration, as described in the variation 3, even when thedegree of deterioration of the developing agent, the image density ofthe respective color is prevented from dropping.

[Aspect Q]

According to any one of aspects L through O, the image forming apparatusincludes an intermediate transfer member to move in a second directionas the transfer target; a secondary transfer device to apply a secondarytransfer bias to secondarily transfer the composite toner image on theintermediate transfer member onto a recording medium; and a secondarytransfer adjuster to adjust a secondary current of the secondarytransfer bias by an amount in accordance with the correction amount ofthe transfer current by the transfer current adjuster.

With this configuration, the degradation of the secondary transfer rateattributed to the adjustment of the primary transfer current issuppressed, thereby enhancing image quality.

[Aspect R]

According to any one of aspects L through Q, the image forming apparatusincludes an environment information obtaining device (i.e., environmentinformation obtaining device 90) to obtain environment informationincluding at least temperature and humidity. The correction amountemployed by the transfer current adjuster is changed in accordance withthe environment information.

This configuration allows adjustment of the transfer current inaccordance with changes in the environment.

[Aspect S]

According to any one of aspects L through Q, the image forming apparatusincludes an electrical resistance detector (i.e., electrical resistancedetectors 80Y, 80M, 80C, 80K) to detect an electrical resistance of apath through which the transfer current flows upon transferring thetoner image from the at least two image bearing members onto thetransfer medium. The correction amount employed by the transfer currentadjuster is changed in accordance with the electrical resistance.

With this configuration, as described in the variation 6, the transfercurrent is reliably adjusted in accordance with changes in theelectrical resistance of the transfer devices (the electrical resistancematerial on the primary transfer current path).

[Aspect T]

According to any one of aspects L through T, the image forming apparatusincludes an image bearing member (i.e., image bearing member 203) torotate in a first direction; a plurality of toner image forming devices(development devices 206M, 206C, 206Y, 206B) to form toner images on asurface of the image bearing member using different developing agentsincluding toners having different volume resistivities; a transferdevice (i.e., primary transfer roller 207) to apply a transfer bias totransfer sequentially the toner images formed on the plurality of imagebearing members onto a transfer medium to form a composite toner image;a developing agent condition detector (i.e., developing agent conditiondetectors 200K, 200C, 200M, and 200Y) to detect a degree ofdeterioration of the developing agents; and a transfer current adjuster(i.e., primary transfer current adjuster 200 a) to adjust a transfercurrent of the transfer bias by a correction amount in accordance withthe degree of deterioration of the developing agents detected by thedeveloping agent condition detector upon transferring at least two tonerimages formed with the developing agents including the toners havingdifferent volume resistivities. The correction amount is differentbetween the at least two toner images.

With this configuration, in the single-drum type image formingapparatus, the transfer current is reliably adjusted in accordance withdifferent volume resistivities of the toners when forming toner imageswith different development agents including toners having differentvolume resistivities on at least two image bearing members.

According to an aspect of this disclosure, the present invention isemployed in the image forming apparatus. The image forming apparatusincludes, but is not limited to, an electrophotographic image formingapparatus, a copier, a printer, a facsimile machine, and a digitalmulti-functional system.

Furthermore, it is to be understood that elements and/or features ofdifferent illustrative embodiments may be combined with each otherand/or substituted for each other within the scope of this disclosureand appended claims. In addition, the number of constituent elements,locations, shapes and so forth of the constituent elements are notlimited to any of the structure for performing the methodologyillustrated in the drawings.

Still further, any one of the above-described and other exemplaryfeatures of the present invention may be embodied in the form of anapparatus, method, or system.

For example, any of the aforementioned methods may be embodied in theform of a system or device, including, but not limited to, any of thestructure for performing the methodology illustrated in the drawings.

Each of the functions of the described embodiments may be implemented byone or more processing circuits. A processing circuit includes aprogrammed processor, as a processor includes a circuitry. A processingcircuit also includes devices such as an application specific integratedcircuit (ASIC) and conventional circuit components arranged to performthe recited functions.

Example embodiments being thus described, it will be obvious that thesame may be varied in many ways. Such exemplary variations are not to beregarded as a departure from the scope of the present invention, and allsuch modifications as would be obvious to one skilled in the art areintended to be included within the scope of the following claims.

What is claimed is:
 1. An image forming apparatus, comprising: anintermediate transfer member to move in a first direction; a pluralityof image bearing members to bear toner images thereon, the plurality ofimage bearing members disposed along the first direction; a plurality oftoner image forming devices to form the toner images on the plurality ofimage bearing members using different developing agents; a primarytransfer device to apply a primary transfer bias to primarily transfereach of the toner images formed on the plurality of image bearingmembers onto a surface of the intermediate transfer member to form acomposite toner image; a secondary transfer device to apply a secondarytransfer bias to secondarily transfer the composite toner image formedon the intermediate transfer member to a recording medium; a controllerto selectively control image forming operation between a first mode anda second mode such that in the first mode the composite toner image isformed using at least two of the plurality of image bearing membersincluding a first image bearing member and a second image bearing memberand after the composite toner image is formed on the intermediatetransfer member the secondary transfer bias is applied to secondarilytransfer the composite toner image from the intermediate transfer memberto the recording medium, and in the second mode the toner image isformed on the first image bearing member used in the first mode which isdisposed downstream from the second image bearing member in the firstdirection and after the toner image is primarily transferred from thefirst image bearing member onto the intermediate transfer member thesecondary transfer bias less than that in the first mode is applied totransfer the toner image from the intermediate transfer member to therecording medium; a first developing agent condition detector to detecta degree of deterioration of a developing agent used to form the tonerimage on the first image bearing member; and a primary transfer biasadjuster to adjust, in the second mode, the primary transfer bias by acorrection amount in accordance with the degree of deterioration of thedeveloping agent detected by the first developing agent conditiondetector so as to reduce a primary transfer current of the primarytransfer bias upon transferring the toner image from the first imagebearing member onto the intermediate transfer member, and to adjust, inthe first mode, the primary transfer bias by a correction amount lessthan the correction amount in the second mode or not to adjust theprimary transfer bias in accordance with the degree of deterioration ofthe developing agent detected by the first developing agent conditiondetector upon primarily transferring the toner image from the firstimage bearing member onto the intermediate transfer member.
 2. The imageforming apparatus according to claim 1, wherein the first image bearingmember is disposed at an extreme downstream end in the first direction.3. The image forming apparatus according to claim 2, further comprisinga second developing agent condition detector to detect deterioration ofthe developing agent in the toner image formed on the image bearingmember used in the first mode other than the first image bearing member,wherein the primary transfer bias adjuster adjusts at least in the firstmode the primary transfer bias for the image bearing member used in thefirst mode other than the first image bearing member by the correctionamount corresponding to the degree of deterioration of the developingagent detected by the second developing agent condition detector, andthe correction amount is greater than that for the first image bearingmember.
 4. The image forming apparatus according to claim 1, furthercomprising a secondary transfer bias adjuster to adjust the secondarytransfer bias by an amount in accordance with the correction amount ofthe primary transfer bias by the primary transfer bias adjuster.
 5. Theimage forming apparatus according to claim 4, wherein the amount ofcorrection of the secondary transfer bias employed by the secondarytransfer adjuster is greater in the second mode than in the first mode.6. The image forming apparatus according to claim 1, further comprisinga secondary transfer bias adjuster to adjust the secondary transfer biasby an amount in accordance with the degree of deterioration of thedeveloping agent detected by the developing agent condition detector. 7.The image forming apparatus according to claim 1, further comprising asecondary transfer bias adjuster to adjust the secondary transfer biasin accordance with an area of the toner image transferred from thesecond image bearing member disposed upstream from the first imagebearing member in the first direction onto the intermediate transfermember.
 8. The image forming apparatus according to claim 7, wherein thesecondary transfer bias adjuster adjusts the secondary transfer bias inaccordance with the area of the toner image and the degree ofdeterioration of the developing agent.
 9. The image forming apparatusaccording to claim 8, wherein the secondary transfer bias adjusteradjusts the secondary transfer bias such that in a case in which thearea of the toner image is less than a predetermined area, a secondarytransfer current is reduced with an increase in the degree ofdeterioration of the developing agent.
 10. The image forming apparatusaccording to claim 1, wherein the first image bearing member bears atoner image formed with a black toner.
 11. An image forming apparatus,comprising: an image bearing member to rotate; an intermediate transfermember to move in a first direction; a plurality of toner image formingdevices to form sequentially and overlappingly a plurality of tonerimages using different developing agents on a surface of the imagebearing member to form a composite toner image; a primary transferdevice to apply a primary transfer bias to primarily transfer thecomposite toner image formed on the image bearing member onto a surfaceof the intermediate transfer member; a secondary transfer device toapply a secondary transfer bias to secondarily transfer the compositetoner image having been primarily transferred on the intermediatetransfer member onto a recording medium; a controller to selectivelycontrol image forming operation between a first mode and a second modesuch that in the first mode the composite toner image is formed using atleast two of the plurality of toner image forming devices including afirst toner image forming device and a second toner image forming deviceand after the composite toner image is primarily transferred onto theintermediate transfer member the secondary transfer bias is applied tosecondarily transfer the composite toner image from the intermediatetransfer member onto the recording medium, and in the second mode thefirst toner image forming device used in the first mode forms the tonerimage which is transferred after the toner image formed by the tonerimage forming device other than the first toner image forming device istransferred and after the toner image formed by the first toner imageforming device is primarily transferred onto the intermediate transfermember the secondary transfer bias less than that in the first mode isapplied to transfer the toner image from the intermediate transfermember onto the recording medium; a developing agent condition detectorto detect a degree of deterioration of a developing agent used to formthe toner image on the first toner image forming device; and a primarytransfer bias adjuster to adjust the primary transfer bias by acorrection amount in accordance with the degree of deterioration of thedeveloping agent detected by the developing agent condition detector soas to reduce a primary transfer current of the primary transfer biasupon transferring the toner image formed by the first toner imageforming device in the second mode, and to adjust the primary transferbias by the correction amount less than the correction amount in thesecond mode or not to adjust the primary transfer bias in accordancewith the degree of deterioration of the developing agent detected by thedeveloping agent condition detector upon primarily transferring thetoner image formed by the first toner image forming member in the firstmode.
 12. An image forming apparatus, comprising: a plurality of imagebearing members to rotate in a first direction; a plurality of tonerimage forming devices to form a toner image on a surface of each of theplurality of image bearing members with developing agents includingtoners having different volume resistivities; a plurality of transferdevices to apply a transfer bias to transfer the toner images formed onthe plurality of image bearing members onto a transfer medium to form acomposite toner image; a developing agent condition detector to detect adegree of deterioration of the developing agents; and a transfer currentadjuster to adjust a transfer current of the transfer bias by acorrection amount in accordance with the degree of deterioration of thedeveloping agent detected by the developing agent condition detectorupon transferring the toner images formed on at least two image bearingmembers, the toner images being formed with the developing agentsincluding the toners having different volume resistivities, wherein thecorrection amount is different between the at least two image bearingmembers.
 13. The image forming apparatus according to claim 12, whereinthe development agent condition detector detects the degree ofdeterioration of the developing agent based on an amount of toner usedby the plurality of the toner image forming devices in a predeterminedtime period.
 14. The image forming apparatus according to claim 12,wherein a plurality of toner patterns having a same length in the firstdirection and different lengths in a width direction perpendicular tothe first direction of the image bearing members are formed in anon-image forming area of the at least two image bearing members foradjustment of image quality, and the development agent conditiondetector detects the degree of deterioration of the developing agent byobtaining a difference in image densities of the plurality of tonerpatterns based on a detection result of the image densities of theplurality of toner patterns.
 15. The image forming apparatus accordingto claim 12, further comprising: an intermediate transfer member to movein a second direction as the transfer medium; a secondary transferdevice to apply a secondary transfer bias to secondarily transfer thecomposite toner image from the intermediate transfer member onto arecording medium; and a controller to selectively control image formingoperation between a first mode and a second mode such that in the firstmode the composite toner image is formed using the at least two imagebearing members including a first image bearing member and a secondimage bearing member and after the composite toner image is primarilytransferred onto the intermediate transfer member the secondary transferbias is applied to secondarily transfer the composite toner image fromthe intermediate transfer member to the recording medium, and in thesecond mode the toner image is formed on the first image bearing memberused in the first mode which is disposed downstream from the secondimage bearing member in the second direction of the intermediatetransfer member and after the toner image is primarily transferred fromthe first image bearing member onto the intermediate transfer member thesecondary transfer bias less than that in the first mode is applied totransfer the toner image from the intermediate transfer member onto therecording medium, wherein in the second mode the transfer currentadjuster reduces a primary transfer current of a primary transfer biaswith an increase in the degree of deterioration of the developing agentupon primarily transferring the toner image from the first image bearingmember onto the intermediate transfer member, while increasing in thefirst mode the primary transfer current of the primary transfer biaswith an increase in the degree of deterioration of the developing agentupon primarily transferring the toner image from the first image bearingmember onto the intermediate transfer member.
 16. The image formingapparatus according to claim 12, further comprising: an intermediatetransfer member to move in a second direction as the transfer medium; asecondary transfer device to apply a secondary transfer bias tosecondarily transfer the composite toner image from the intermediatetransfer member onto a recording medium; and a controller to controlimage forming operation in a control mode in which the toner images areformed on at least three image bearing members including at least thefirst image bearing member and the second image bearing member and areprimarily transferred onto the intermediate transfer member such thatthey are superimposed one atop the other to form the composite tonerimage which is transferred onto the recording medium, wherein in thecontrol mode the controller controls the image forming operation suchthat with an increase in the degree of deterioration of the developingagent having a first toner used in the toner image formed on one of theat least three image bearing members disposed at a downstream side inthe second direction of the intermediate transfer member to express afirst color in an image, a ratio of use of other toners used in thetoner images formed on at least two other image bearing members upstreamfrom the image bearing member used for the first color is increased toexpress the first color without using the first toner.
 17. The imageforming apparatus according to claim 12, further comprising: anintermediate transfer member to move in a second direction as thetransfer target; a secondary transfer device to apply a secondarytransfer bias to secondarily transfer the composite toner image on theintermediate transfer member onto a recording medium; and a secondarytransfer adjuster to adjust a secondary current of the secondarytransfer bias by an amount in accordance with the correction amount ofthe transfer current by the transfer current adjuster.
 18. The imageforming apparatus according to claim 12, further comprising anenvironment information obtaining device to obtain environmentinformation including at least temperature and humidity, wherein thecorrection amount employed by the transfer current adjuster is changedin accordance with the environment information.
 19. The image formingapparatus according to claim 12, further comprising an electricalresistance detector to detect an electrical resistance of a path throughwhich the transfer current flows upon transferring the toner image fromthe at least two image bearing members onto the transfer medium, whereinthe correction amount employed by the transfer current adjuster ischanged in accordance with the electrical resistance.
 20. An imageforming apparatus, comprising: an image bearing members to rotate in afirst direction; a plurality of toner image forming devices to form aplurality of toner images on a surface of the image bearing member usingdifferent developing agents including toners having different volumeresistivities; a transfer device to apply a transfer bias to transfersequentially the toner images formed on the image bearing member onto atransfer medium to form a composite toner image; a developing agentcondition detector to detect a degree of deterioration of the developingagents; and a transfer current adjuster to adjust a transfer current ofthe transfer bias by a correction amount in accordance with the degreeof deterioration of the developing agents detected by the developingagent condition detector upon transferring at least two toner imagesformed with the developing agents including the toners having differentvolume resistivities, wherein the correction amount is different betweenthe at least two toner images.